Free Standing Appliance
After the initial Earth Continuity and Isolation Tests have been carried out it is time to start working on the appliance, before we can do this, we need to make the machine safe to work on by safely isolating it from the power supply. We do this on a free-standing appliance by using a lock-off device on the plug.
See the image below.
The whole point of devices like this is to prevent anybody from plugging the appliance back into a live power outlet whilst you are working on it and also to make it safe for the customer in case you have to return to your vehicle for any reason. It is also worth mentioning that some companies or manufacturers will require you to leave a lock-off device in place if you have identified an unsafe appliance and are unable to repair it on that visit.
Hard Wired Appliance
After you have performed an Earth Continuity and an Insulation Test you will need to safely isolate the appliance from the electrical supply. The principle is the same as a free-standing appliance but the method is different.
Most hard-wired appliances will have an isolation switch somewhere near the machine as shown in the image below, but you cannot take for granted that it is working properly and it would be very easy for somebody to switch it on whilst you are working on the appliance. It is also possible the isolation switch might have been wired incorrectly by a weekend D.I.Yer.
What we need to do in this instance is locate the customer’s consumer unit and then identify the correct circuit on the fuse box which is usually labelled (e.g., cooker or hob).
Once we have done this, we would then use an MCB/RCD lock off device as shown in the image below.
The idea of this device is to lock the MCB/RCD in the off position, the engineer would set their combination code and this would prevent anybody from switching the circuit back on whilst you are working on the appliance.
There aren’t any lock-off devices available for older consumer units that use fuse wire (see image below). The best way to safely isolate these would be to identify the circuit you are working on and remove the specific cartridge from the consumer unit and place it in your toolbox, that way the circuit could not be made live again whilst you are working on it.
Once you have safely isolated the appliance from the mains you then need to prove that the machine you are about to work on is electrically dead. There are two ways you do this, you could use a Volt Stick (Volt Pen) to prove the mains cable and terminal block are no longer live, or the preferred method would be to use a multimeter on the AC voltage setting to prove there is no longer a live feed across the terminal block by first testing from live to earth, then from neutral earth and finally from live to neutral. Once you have proved it is no longer live then you can start your fault finding and repairing process.
These safety measures may seem extreme or unnecessary but please remember for procedures like this to become part of safe working practices then somebody will have been severely hurt or possibly even fatally injured by an appliance that wasn’t made safe. It is also worth mentioning the companies and manufacturers that make safe isolation part of their working practices will very often initiate disciplinary or suspension procedures if you or the customer got injured by not safely isolating the appliance.