The costs of fitting d.c braking is approximately £125 for parts + £125 for electrical labour costs per motor, although this is dependant on the size of the motor that will be appropriate to the machine.
If the machine isolator is operated then power to the entire machine, including the braking unit, is cut off before the braking cycle has completed so the braking effect is lost. This has caused ‘run down’ accidents on machines fitted with DC injection braking when operators have approached the machinery for setting, not realising that the cutters were still rotating. It is therefore vital that the machine E-stop is always used to allow the braking cycle to complete before isolating the power supply.
The wiring to a d.c. brake should be such that actuating an E-stop, either a local or remote one, sends the d.c to the brake and stops the machine, i.e. this action is not prevented by how it is wired.
If the electrical supply to the factory/workshop etc. is known to be unreliable then to ensure that the d.c brakes still work either a battery backup or emergency a.c. supply of some sort should be considered (the d.c. brake control unit acts as a rectifier). However, in practice, although not unknown, battery backup is rare as it is very expensive, both financially and in space requirements and any battery pack would need to be able to meet the demands of all the machines that needed to be braked.
As long as the manual brake is maintained and works properly by quickly bringing the saw to a stop, there is no need to fit additional braking. If however the saw gets a lot of use and operators have been known not to use the manual brake then the risk assessment for the saw should be reviewed and consideration given to fitting electrical braking.
A spring or pulley system that returns the saw into a safe protected area can be fitted, as shown in cross-cut saw that will stop blade contact whilst it is running down. However, you will need to consider in your risk assessment how often the saw is used and by who. If lots of people use the saw and it has a long run down time, there may be a chance that another user pulls out the saw carriage not knowing that the blade is still rotating. If this is the case then braking may be a better option.
Many companies use trenching heads on radial arm cross cut saws. The increased weight and momentum of trenching heads causes longer run down times and this needs to be taken into account in your risk assessment (see previous question). If braking is to be used it should be able to cope with the heaviest blade in regular use and bring the saw to a complete stop before it cuts out. Note: all such tools must be ‘chip-limited’.
When working with thicker timber the machine feed table is often lowered sufficiently to allow ease of access to the cutters. The machines typically have a long run down period which increases the risk when making adjustments or clearing waste material.