The purpose of this Information Sheet is to provide guidance to users of Ex certified High Voltage (HV) motors in use in hazardous areas on offshore oil and gas installations located on the UKCS.
The objectives are (1) to introduce information on the ignition risks of Ex certified high voltage motors used in hazardous areas, (2) to provide current information on the ignition risks of these motors, (3) to provide advice on the management of motors constructed to older, revised or withdrawn standards, (4) to provide information as to how the ignition risks of these motors are currently managed by users, and (5) to provide information on the actions required by users in the management of these ignition risks of HV motors.
Following incidents associated with high voltage (above 1000V rms) cage induction motors, the current European harmonised ATEX standards, BS EN 60079-7: 2007 for Ex e and BS EN 60079-15: 2005 for Ex n (nA), have been updated. These revisions incorporate major changes which recognise that rotor and / or stator sparking may occur during starting. New information [IEEE IAS PCIC Conference, Weimar 2008, ISBN: 978-3-9523333-1-01] indicates that if stator winding discharges are present then they are likely to be present all the time i.e. during starting and running conditions (so that existing control measures such as pre-start purge may not be sufficient).
New motors placed on the EU market certified by the manufacturers to the current standards involve application-specific input from the user; special precautions may be required by the user to ensure that these machines remain safe in operation. Existing motors correctly certified to older standards may nonetheless require some of those special precautions to be put in place by the user in order to render their risks as low as is reasonably practicable (ALARP).
Gas ingress via machinery driven by the motor was implicated in some of the incidents, and users should avoid configurations which allow this; this issue may also be relevant to Ex p certified machines.
Note that some users manage ignition risk by operational procedures to ensure that there has not been a gas release in the area prior to starting the motor; this procedure may be less appropriate than an automatic interlock. Such procedures will not cover stator sparking which may occur during running.
Also note that recent experience indicates that cooling failure has resulted in motors overheating and in doing so presented an ignition risk. Overheating of motors might also occur owing to non-sinusoidal supplies (i.e. with harmonic distortion). Users should understand these risks and put in place effective control measures.
Ex d (flameproof) certified motors (category 2), constructed to BS EN 60079-1:2007 or earlier standards, are designed to ensure that any internal explosion will not damage the housing, and that hot combustion products will be cooled as they exit the machine via clearances (called the flame path) in the housing assembly. On entering the surrounding atmosphere, these gases are sufficiently cooled so as to prevent auto ignition of any surrounding hazardous atmosphere. The casing of Ex d machines needs to be heavily built in order to withstand the explosion overpressure, thus large HV Ex d certified machines tend to be very expensive and are not common offshore. In addition to this the Ex d concept caters for the possibility of internal sparking to occur, so these motors are considered no further.
Ex p (pressurised) certified motors (category 2), constructed to BS EN 60079-2:2007 or earlier standards, ensure that a safe gas at a pressure above the surrounding atmosphere is used to preclude the ingress of any surrounding hazardous gases, so that internal ignition sources cannot be effective. However, the flow rate of the protective safe gas may not be sufficient to dilute any internal release of gas (e.g. gas entrained in the lubricating oil) to below its lower explosive limit, unless specifically certified in relation to an internal gas release. The additional cost of providing and maintaining the pressurising gas supply, instrumentation and interlocks required by the standard makes Ex p machines relatively expensive. The Ex p concept is likely to prevent internal ignitions and might be used to further protect HV Ex e, N and n motors.
Pre-start purge (one of the special measures that may be applied to HV Ex e, N or n motors) as the name implies is solely designed to purge clean air (or an inert gas from portable bottles) through the motor enclosure to remove any residual potential flammable gas from within. Its sole purpose is to prevent ignition risk due to rotor or stator sparking during starting. Air flow sensors and timers may monitor this process before allowing the starting of the motor. Once the motor is started no further air flow monitoring is provided.
Pre-start purging may not be sufficient to prevent ignition risks owing to stator sparking which might also be present during running conditions. This has only recently been recognised and users should assess their ignition risk control measures taking this into account.
Ex N and Ex n (non-sparking) certified motors (category 3) and Ex e (increased safety) certified motors (category 2) are essentially standard 3-phase motors totally enclosed fan cooled (TEFC) with certain modifications to reduce the probability of sparking. These motors are widely used and are the main subject of this note. Some motors are fitted with external heat exchangers. Most large HV motors are located within a zone 2 hazardous area, so category 3 certified Ex N or Ex n type HV motors are the most common.
Type Ex N, Ex n and Ex e motors should not be used where the probability of an explosive gas release cannot be totally disassociated with the start sequence as an independent event e.g. the oil seal systems of gas compressors are known to produce such releases during starting and should be subject to assessment. Seal or lubricating oil systems shared between a motor and its driven compressor are not recommended.
In the UK, during the period 1984 to 1991, six incidents were recorded, including five explosions, three offshore and two onshore. These incidents involved motors operating at 6.6kV and above and were confined to type Ex N motors. In the incidents involving explosions, flammable gas had entered the motor enclosure either by an external gas release being drawn into the enclosure following motor cool down, or via other routes such as common lube oil systems.
The Department of Energy issued Safety Notice 17/90 in 1990. This was revised and reissued by HSE in March 1995. As safety notices have a finite life, it has subsequently been withdrawn.
An offshore incident in 2003 again highlighted the problem. This incident resulted from a substantial leakage of gas compressor seal oil, allowing gas to migrate from the compressor through the seal oil system into the common compressor and motor lube oil system, and then into the motor enclosure. On starting the motor an explosion occurred within its enclosure. This motor was not fitted with the pressurisation or pre-start purging system required as special conditions of its certification (indicated by an X on the certificate).
More recently, there has been an incident where a motor overheated beyond its certified temperature 'T' rating, and emitted large quantities of smoke (and was badly damaged).
Gas may be drawn into a motor enclosure from a release in the surrounding area, for example when the motor cools down and creates a reduced pressure within the motor enclosure. There is no practical way of sealing the internal motor enclosure.
Ex N, Ex n and Ex e motors which have cooling provided by an external heat exchanger have no means of dispersing any gas which does enter the motor enclosure, as they circulate a largely trapped volume of air. Therefore, any flammable mixture that occurs within the motor enclosure is likely to remain there for a considerable time.
Another possible source of gas ingress occurs where the motor drives a load which handles a flammable fluid, e.g. a gas compressor; there is a potential path for gas to enter the motor enclosure in contaminated lube oil where common lube/seal oil systems are used. Although Ex N, Ex n and Ex e motors should not cause sparking the recent standards acknowledge the potential rotor and stator winding sparking on motors >1kV and require the user to consider additional control measures e.g. Ex p protection.
Users should ensure that the motor lube oil cannot be contaminated with gas. Note that this caveat applies equally to Ex p motors, unless the continuous purge rate is sized for an internal source of gas release from the lube oil. In some cases, motor pedestal bearings provide a break where gas can dissipate from lube oil into the surrounding atmosphere, rather than being released within the machine, though the effectiveness of this measure is not easily quantified.
Ignition of any flammable atmosphere within the machine has the potential to cause an internal explosion which may damage the machine. However, ignition also has the much more serious potential to ignite any flammable atmosphere surrounding the machine and as such is a major accident hazard.
The package of measures for the control of ignition risk presented by electrical apparatus in hazardous areas on offshore installations is considered to be a Safety Critical Element (as defined by the Offshore Installations (Safety Case) Regulations 2005).
Several sources of ignition need to be considered for cage motors:-
High surface temperatures can be controlled by the manufacturer by derating and attention to hot spots, etc. Embedded temperature sensors can be used to provide an active (instrumented) high temperature alarm and trip function. If the machine is stalled, it will heat up much further and more rapidly than in normal operation, and protection for the stalled condition can be provided by the user by electrical protection relays. The functionality and integrity of all such active protective functions should be formally managed by the user (e.g. via IEC 61508 / IEC 61511 methods).
Breakdown of the air in the air gap (between stator and rotor) caused by the field gradient is unlikely; a typical gap is of the order of 5mm, a typical voltage is 6kV, so a typical electric field gradient is 1.2MV/m, somewhat less than the breakdown voltage of dry air, 3MV/m. Also, the stator windings are insulated, so that only part of the stator winding voltage appears across the air gap; incendive breakdown of the air in the air gap is not to be expected and no particular precautions are employed. However, sparking at the surface of the stator or rotor will be visible in the air gap.
Stator spark activity consists of localised discharges across part of the stator winding insulation surface (rather than gross breakdown of the insulation), due to the high electric field gradient. Localised discharges are also called partial discharges (PDs), and may be more of an issue where the surface is contaminated or wetted by condensation. Where the contamination causes increased capacitance, a higher stored energy across the surface is available to PDs, which may then have more capability to be incendive (note that the minimum ignition energy of methane is very small, about 250 microJoules, see Hattwig and Steen2). Another possible mechanism for surface contamination to increase the likelihood of discharges is a lower breakdown voltage. Recent information indicates that these stator winding surface discharges may occur during running as well as during start up; this is to be expected since the voltage gradients during running are similar to those during start up, apart from switching transients.
Stator spark activity is controlled by the manufacturer by avoiding points of very high electric field gradient (e.g. by the application of semi-conducting tape), attention to clearances, etc. Cleaning of the stator winding surface may also be required and is a responsibility of the user. Anti condensation heaters may be provided to eliminate condensation on the stator (and rotor) surface. Anti condensation heaters may be provided to eliminate condensation on the stator surface and connecting cables.
Rotor sparking may take place during starting and occurs between the rotor bars and rotor core. It is caused by movement of the rotor bars on starting, due to high forces at start-up, either within the slots or around the end of the core in the area where the bar is shaped. These movements interrupt the circulation of stray currents through the magnetic core material (unless the rotor bars are insulated, which is not common). Interruption of such stray currents can be incendive, so bars need to be fully locked into place (or insulated) by the manufacturer.
Static charges can accumulate on any unearthed parts of the machine, typically the cooling fan and associated parts. This static is dissipated by equipotential bonding and a choice of conducting materials, applied by the manufacturer; unlike electric shock safety bonding/earthing, this does not need a low resistance path; anything below 10Mohm is likely to be effective in dissipating static charges.
Circulating currents in the machine as a whole are inevitable, but sparking is eliminated by sound, low resistance connections (bonding straps) between conductive parts such as the machine casing components, terminal box, etc. The manufacturer should supply, and the user should maintain, these bonds.
Mechanical rubbing of the rotor or fan is controlled by attention to air gap clearances, fan/housing clearances, bearing slop, etc. The manufacturer should correctly construct the motor, and the user should maintain these aspects.
In low voltage (LV) (i.e. up to 1000V) ac induction motors, stator discharge or cage rotor sparking is generally considered to be a low risk.
Where the manufacturer specifies special conditions of use (indicated by an X on the certificate), the user is responsible for employing and maintaining those special conditions. Special conditions are of two sorts, (1) to detect gas within the machine, and (2) to exclude gas from the machine.
The risk of ignition during starting. Gas detection within the machine enclosure may be provided, so that machine starts may be avoided where gas is present within the machine.
Pre-start purge may be adopted for Ex e, N or n motors at risk of incendive sparking, and is designed to purge clean air through the motor enclosure to remove any residual potentially flammable gas. Its purpose is to prevent the risk of explosions due to rotor sparking during starting; air flow sensors and pre-start timers may monitor this purge process before allowing the application of the HV electrical supply, or control may be achieved by procedures. Once the motor is started, no further pre-purge flow is provided. A suitable purge connection point should be provided by the manufacturer. The purge gas supply is provided by the user, from a fixed supply or from portable gas bottles. (Nitrogen is sometimes used instead of using clean air).
Ignition risks owing to stator sparking may also occur when the motor is running; this phenomenon has only been acknowledged relatively recently. Note that pre-start purge does not offer any protection, but suitable special measure might be to pressurise the motor enclosure continuously in order to prevent the ingress of a potentially flammable atmosphere; this arrangement should be interlocked with pre-start and post shut-down timers and pressure/flow measurement, but may not fully comply with the standard for Ex p apparatus.
Standards for explosion protected apparatus have a long history, and have developed over some decades to give more detailed advice and to achieve higher levels of safety. For example, BS 5000:Part 16:1985 Rotating electrical machines with type of protection Ex N defines requirements for machines which are non-sparking in normal operation and have surface temperature limitations. This standard discusses sparking associated with joints between rotor conductors and short circuiting rings, but does not mention the potential for rotor sparking caused by the interruption of stray currents through the rotor magnetic core material; it does not recognise stator sparking at all. However, some of today's live issues were covered in the 1985 standard, albeit briefly, e.g. non-sinusoidal supplies. The 1993 version of BS 5000 recognises that electrical discharges are associated with heavy contamination of high voltage windings, and mentions intermittent contact between bare rotor conductors and other metal parts, but without mentioning that this is most likely during starting.
In 1997, CENELEC published DD ENV 50269 Assessment and Representative Testing of High-Voltage Machines in response to concerns over HV motors, including the incidents noted above. This standard specified additional assessment and representative testing of electrical machines which are at risk of incendive sparking. These requirements have now been incorporated into recent revisions of the harmonised standards BS EN 60079-7: 2007 Equipment protection by increased safety "e" and BS EN 60079-15:2005 Construction, test and marking of type of protection "n" electrical apparatus (BS 5000 has been withdrawn). The assessment is based on environmental conditions, frequency of starting and inspection interval, factors within the control of the user. Note that Ex n HV motors should not be used where the possibility of a gas release cannot be totally dissociated from the start sequence.
For Ex n motors started at least once per week, and for Ex e motors where the starting current is not limited to 300% of the rated current, the standards require a rotor cage potential sparking risk assessment which considers rotor cage ignition risk factors. This risk assessment takes into account the rotor cage construction, the number of poles, rated output, radial cooling ducts in rotor, rotor or stator skewed, rotor overhang parts and the temperature class/ limiting temperature. From these characteristics, risk factors are assigned and if the total sum of the factors determined is greater than 6, the machine or a representative sample should be type-tested. The HV motors under discussion may exceed this limit. Type tests (usually performed by the manufacturer) involve test starting a machine in an explosive gas mixture at the rated supply voltage in order to show that no incendive sparking occurs.
Alternatively, the machine should be constructed to allow special measures to be employed to ensure that its enclosure does not contain an explosive gas atmosphere during starting. The standards suggest that this may include pre-start ventilation or the application of fixed gas detectors; some manufacturers have supplied systems that continually pressurise the motor enclosure (although these are not intended to be compliant with BS EN 60079-2 for Ex p equipment). The machine marking/certificate should include the sign "X" to indicate that special conditions of use are required. The motor manufacturer is responsible for carrying out the risk assessment and providing connections for the special measures if required by the assessment, though the user is responsible for using them.
For Ex e machines, a stator type test is required. For Ex N and n machines, a potential stator winding discharge risk assessment process is applied; when required, either a type test is conducted by the manufacturer or special measures should be employed by the user.
The user is responsible in completing this risk assessment which is based on rated voltage, average starting frequency, time between detailed inspections and environmental conditions. If the total ignition risk factor exceeds 6 for Ex n motors (which for HV motors it commonly will) the machine, or a representative sample, should be tested in an explosive gas mixture at 1.5 x the rated supply voltage for 3 minutes to show that no incendive sparking occurs, or alternatively special measures should be employed as described above. For Ex e motors, the stator type test at 1.5 x rated voltage for 3 minutes, fittings for special measures, and anti-condensation heaters are compulsory. Some of the risk factors are application-specific, and require the user to provide information to the manufacturer. The in-service application of special measures is the responsibility of the user.
The offshore HV motor incident in 2003 revealed that the user had failed to recognise that the machine marking included the sign "X" and failed to install pressurisation or pre-start purge.
A representative machine and connecting cables should be tested in an explosive test mixture and subjected to 10 defined voltage impulses of not less than 3 times peak phase to earth voltage. This may offer some assurance as regards use of the motor on a non-sinusoidal supply (which may have several switching transients per cycle of the supply) as well as normal starts, which involve a single switching operation.
For motors driving high inertia loads, the ignition tests are representative only of normal operation away from torsional resonance. Users of HV motors should consider machine applications where torsional resonance of the complete motor/load drive train might occur, as this may increase the probability of sparking, e.g. because of higher forces on the rotor bars. Additional testing or the special measures described above may be required.
The ignition tests described above are representative only of normal operation with starts from the stationary condition and are not adequate for motors intended for auto re-start (an out of phase re-start may result in higher forces on the rotor bars, and a longer start up phase which may cause additional heating). Users of HV motors should consider machine applications where out-of-phase re-starting might occur, as this may increase the probability of sparking. Additional testing or the special measures described above may be required.
Pressurisation ensures that a safe gas, at a pressure (normally around 6 mm water gauge) above the surrounding atmosphere, is used to preclude the ingress of any surrounding hazardous gases. Internal release of gas is a special case, for which motors are not usually certified. Care should be taken to prevent overpressure as this could cause the motor enclosure to distort and become damaged. A pressure relief device is usually fitted to prevent this occurring.
In most cases, prior to starting the motor, clean air from a non-hazardous area is purged through the motor enclosure for a defined cycle time to remove all residual gas. Once the purge cycle is complete, the motor is allowed to start. The clean air supply continues to pressurise the motor enclosure above atmospheric pressure to prevent gas ingress from the surrounding area during motor running. Pressurisation should continue for a defined time period after the motor has stopped to prevent any surrounding hazardous atmosphere being drawn into the motor during the cooling down period whilst motor internal temperatures may remain above the T class rating.
This pressurisation (monitored via a pressure sensor) and air flow (monitored via a flow sensor) of the motor enclosure should be monitored throughout motor operation. Loss of pressure or flow should alarm and trip the motor supply as required by EN 60079-14:2008 (Table 10). As Ex p motors normally lack an internal source of release (see above re gas ingress), an alarm should be generated for zone 2 applications (and may also be generated for zone 1 applications, if necessary); for zone 1 applications, the motor should be tripped (unless automatic switch-off would introduce a more dangerous condition, when other precautionary measures should be taken, for example duplication of the protective gas supply). If the alarm operates, immediate action should be taken, for example to restore system pressure. For zone 2 applications, restoration of pressurisation should be completed as soon as possible, but in any case within 24 hours. During the time that the pressurisation is inoperative, action should be taken to avoid the entry of flammable material into the motor enclosure.
Apparatus within the enclosure suitable for the external zone need not be switched off when pressure fails (e.g. Ex certified temperature sensors).
An offshore incident in 2006 highlighted another concern with regards to HV motors; a large HV motor overheated owing to a failure of its cooling function and the motor ran to destruction, reaching a temperature well in excess of T3 (=200 deg C); copious quantities of smoke were emitted, which was dangerous for personnel in the vicinity, but had flammable gas been present, this high temperature could have caused auto ignition.
Many motors are fitted with integral cooling fans (i.e. cooling fans directly connected to the non drive end of the rotor); this type of fan inherently circulates air whilst the motor is running, so the cooling function has no obvious failure mechanism. Some HV motors have an internal fan which circulates a volume of trapped air from within the motor enclosure through a heat exchanger which removes heat; this design reduces the possibility of external air being drawn into the motor enclosure and hence may reduce the risk of hazardous gases being present (but tends to trap any which is present). The cooling medium for the heat exchanger may be water or external air. Air cooling systems typically consist of an integral external cooling fan which inherently circulates air or alternatively by separate driven (e.g. electric) multiple redundant fans. When separate air or water cooling systems are used the motor starting circuits should include interlocking to ensure that prior to starting the motor, the cooling medium is flowing. Cases have been found where once the HV motor has started, there is no interlock to stop the motor should the external cooling system then fail. The investigation into the above incident found that this executive trip action had not been wired to the motor starter, and when the external electric cooling fans failed, the HV motor ran to destruction. For motors not fitted with integral fan cooling, it is important that temperature sensors are embedded within the windings to provide a high temperature alarm function (typically 850C) and high-high trip function (typically 1050C) where class B insulation limits are observed. Application and maintenance of these instrumented protective functions is a user responsibility.
Many offshore installations have equipment such as variable speed drives (e.g. down-hole pumps) which are liable to produce harmonic distortion in the electrical supply system. Such harmonic distortion has the potential to adversely affect other connected electrical equipment. The typical effect on the machine is an increase in running temperature.
AC motors rated for use on a power supply of fixed frequency supplied from an a.c. generator (whether local or via a supply network) should be suitable for the following requirements:
Suitable for operation on a supply voltage having a harmonic voltage factor (HVF) not exceeding:
Note: Design N motors (EN60034-12:2002) refers to the normal starting torque three-phase cage induction motors intended for direct-on-line starting, having 2, 4, 6 or 8 poles and rated from 0.4kW to 1600kW.
The harmonic distortion (voltage waveform) should not exceed:
In circumstances where HV motors are subject to harmonic voltage distortion in excess of the limits stated in the above standards users should reduce this distortion.
Impulses on the supply should also be considered; non-sinusoidal supplies may contain switching transients which may have high peaks and rapid edges which may promote sparking. Current standards for Ex e and Ex n motors require an impulse ignition test to be carried out, and this test adequately addresses this hazard. Older standards do not call for this test, so users should review this potential effect of harmonic distortion for motors certified to older standards.
Users should consider the following actions in the management of ignition risks presented by HV Ex e, N and n type motors located in a hazardous area:
1) Determine the current status of all HV Ex e, N and n type motors installed, together with their control measures. These findings should be compared with the changes in the Ex standards and determine whether any further actions are required. For motors built and tested to the new standards the manufacturer should design and test a sample of their rotor and stator construction so that they do not create a potential ignition risk. Users will need to check that any motors supplied have been appropriately designed, tested and are suitable for their application.
2) Check rotor and stator discharge risk assessments and ensure that any control measures have been completed in accordance with the motor application and location.
This risk assessment method might be applied to all existing HV motors. Users should ensure that the interval between disassembly, cleaning, the examination of windings, the degree of IP protection and the environmental conditions are in accordance with this risk assessment.
3) Check the motor Ex certification and if this includes the sign "X" ensure that any special conditions e.g. pressurisation or pre-start purge have been installed.
4) Consider machine applications:
5)For motors fitted with pre-start purging consider that pre-start purging is unlikely to prevent ignition risks owing to stator sparking (since can occur when the motor is running).
6) For Ex p motors consider the flow rate of the protective gas may not be sufficient to dilute any internal release of flammable gas (e.g. entering the motor via common lube oil systems) to below its lower explosive limit.
7) Users should check that any control measures or procedures for monitoring the loss of gas compressor seal oil should check that these measures or procedures are being completed and that they are appropriate.
8) Users that manage these potential risks by having operational procedures in place should check (1) that appropriate procedures exist, (2) that offshore operations personnel are fully aware of these procedures, and (3) that offshore operations personnel understand why these procedures are in place and under what circumstances the motors are and are not allowed to start.
9) Where an installation has had a change of owner, users should ensure that procedures that were in place have been transferred over and that personnel are fully aware of these procedures. Typically, such procedures require the motor enclosure to be ventilated if it is considered that the internal spaces may contain a potentially explosive atmosphere (e.g. after a gas release within the area where the motor is operating), to disperse any gas within the motor enclosure prior to start. Users should confirm that this procedure is in place, is understood, is carried out and is fit for purpose.
10) Users should routinely test Ex protective functions (e.g. motors fitted with pressure relief, pressure sensors, air flow sensors and Ex protective control systems) and ensure that their alarm and trip functions are correctly installed.
11) For motors fitted with external cooling systems users should consider the Ex motor protective functions e.g. winding high temperature trips and ensure that these are connected into the motor control system and are routinely tested.
12) For non-sinusoidal supplies where the power supply to the motor contains significant harmonics which exceed the requirements of the standard to which the motor is certified, the additional temperature rise in service should be assessed and protection against over temperature considered. More importantly, users should put in place appropriate control measures (e.g. harmonic filters) to reduce the harmonic content of the power supply to an acceptable level.
The Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations 1995, Regulation 9(2)(d) requires measures to control electrical or other sources of ignition.
The Electricity at Work Regulations 1989, Regulation 6(d) requires measures to control electrical sources of ignition.
The Provision and Use of Work Equipment Regulations 1998, Regulation 12(2)(e) requires measures to control unintended explosions.
This information sheet contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do.