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Safe operation and maintenance of unearthed electrical distribution system

Offshore Information Sheet No. 2/2011

(Issued January 2011)



1. This information sheet is for operators of installations that use unearthed electrical distribution systems (identified as IT systems in BS 7671). It reminds them of the key safety and operational features of such electrical systems. IT systems are normally provided on FPSO's, MODUs, drilling rigs and flotels. This sheet replaces Safety Notice 9/2005 (issued November 2005) which is now withdrawn.


2. There have been several electrical accidents resulting in injury. The accidents have often occurred on IT electrical distribution systems. Subsequent inspections by HSE have identified that earth faults are often not being traced and cleared sufficiently quickly, resulting in some IT electrical distribution systems being operated with multiple earth faults. Operation of IT electrical distribution systems with multiple earth faults present increases the risk of electrical shock, burns and fire. The high level of electrical supply availability, which is a key safety feature of IT electrical distribution systems, is also compromised by earth faults.

Action required

3. Dutyholders should:

Tracing faults

4. Insulation monitoring arrangements play a crucial role in assisting with the tracing of earth faults. The 3 lamp system and the basic insulation level monitor system only provide limited assistance to maintenance technicians. More sophisticated systems are now available which facilitate the rapid tracing of earth faults. As well as reducing the time that a fault remains present, the use of these systems also minimises the requirement for supply disconnection and technician intervention on energised electrical equipment.

Residual current devices (RCD's) for protection against electrical shock and for tracing earth faults in final circuits

5. RCDs have generally been regarded as only being effective on solidly earthed systems. However, RCDs can have a role in protecting against electrical shock and can assist with the tracing of earth faults when used on IT electrical distribution systems. The IT electrical distribution system insulation impedance to earth will often be at a value low enough to allow an RCD to function when an earth fault occurs. This is particularly so with electrical distribution systems that incorporate harmonic line filters.

6. For protection against electrical shock it is recommended that a 30 ma rated RCD be incorporated in IT electrical supplies to portable electrical equipment, unless other risk reduction measures have been taken, e.g. use of a reduced voltage.

7. On the occurrence of a first earth fault, an IT electrical distribution system becomes earthed, via the fault impedance. The occurrence of a second earth fault may not cause the fuse or circuit breaker protection to operate if the fault impedance restricts the fault current to a level below that which is required to operate the protection. When other earth faults occur, a situation can then develop that results in an IT electrical distribution system having multiple earth faults present.

8. Using an RCD, which requires a much-reduced fault current to operate, is more likely to disconnect a circuit that has developed an earth fault than using a fuse or circuit breaker. Incorporation of RCD's into final circuits that are susceptible to earth faults, such as deck lighting, can therefore assist in the tracing and clearing of faults.

Danger of high fault currents

9. A first earth fault does not result in automatic disconnection for an IT electrical distribution system. However, a subsequent second fault, in certain situations, can give rise to destructive and hazardous short circuits unless cleared quickly by the upstream electrical protection. An example is with LV switchboard incomers where, in some cases, a short circuit fault current may be as high as several thousand amperes and the upstream protection is not able to clear the fault sufficiently quickly to prevent damage.

10. Incomers on a conventional earthed TN system would typically have earth fault protection with settings designed to clear the fault before damage could occur. The risks of such faults occurring on an IT system should be assessed and, where necessary, appropriate action should be taken to reduce the risks. Such a short circuit fault has occurred on an FPSO resulting in severe injury to the technician who was engaged in attempting to trace an earth fault.

Competence to work on IT electrical distribution systems

11. IT electrical distribution systems have significant technical differences to the more conventional earthed TN electrical distribution systems encountered onshore and on offshore production installations. Maintenance technicians responsible for IT electrical distribution systems should be competent. It is therefore important to assess a technician’s competence in relation to the key safety features of IT electrical distribution systems and provide any necessary instruction and training that is identified.

Relevant legal requirements


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.

Updated 2011-10-10