FCU fault?

[Iddy]

Evening everyone

Hope everyone is well.

Just to make it clear i am not a qualified electrician.

A friend of mine who is a Gas service engineer has mentioned that he went out to a job yesterday to upgrade an old boiler for a new one.
He says he he had switched of the FCU and removed the fuse before he started his work. Upon him removing the old cable feeding the old boiler he got an electric shock?

How would this be possible ? he thinks there is a fault in the FCU

Can anyone provide their expert advice as to why this would happen

Many thanks

 

[davelerave]

the boiler may not be isolated properly

his own fault for getting a shock

 

[Des 56]

It could have been a single pole FCU
The neutral would have remained connected when it was switched off
The neutral may have had a potential difference to the earth on the boiler and if he bridged the neutral and earth he could have felt a shock

Another way could be, as above, but the live and neutral reversed in the switch
When switched off,the neutral may have been switched and the live still connected

He meeds to isolate the supply and test at the boiler with a voltage tester before dipping his fingers in the connections

 

[ezzzekiel]

If he had bothered to test for dead then he wouldnt of recieved a shock - pretty much serves him right, and he shouldnt automatically blame the fcu without any prior testing.

 

[Iddy]

Gents

I really appreciate the replies,

makes complete sense now its obvious to me he did not do a safe isolation from you guys have described.

Thanks

 

[tical]

It's also possible, the supply to the boiler is fed from a central heating controller elsewhere in the house which may provide a switched supply as well as a permanent feed. This may not be isolated before it connects to the boiler, so pulling the fuse on the connection unit wont isolate all supply. As the other guys say, isolate at the fuse board, and prove the supply is dead with a meter. Every time !

 

[MarkieSparkie]

It's also possible, the supply to the boiler is fed from a central heating controller elsewhere in the house which may provide a switched supply as well as a permanent feed. This may not be isolated before it connects to the boiler, so pulling the fuse on the connection unit wont isolate all supply. As the other guys say, isolate at the fuse board, and prove the supply is dead with a meter. Every time !

Proving dead should be done with an approved voltage test lamp or two-pole voltage detector together with a proving unit as recommended in HSE Guidance Note GS38. Use of multimeters and non-contact voltage sticks have resulted in a number of fatalities.

 

[BlueToBits]

There is a possibility that the main bonding is missing or inadequate and the boiler was providing the only path through it's supply cable for earth current from a fault elsewhere in the house.

 

[telectrix]

the obvious answer is that only the load side of the FCU was dead, and he touched the line on the feed side

 

[BlueToBits]

the obvious answer is that only the load side of the FCU was dead, and he touched the line on the feed side

Would have to agree. I missed the key phrase "Gas service engineer". ....Must always read and re-read the question

 

[telectrix]

maybe the ambient light was too bright for him to see the neon on his state of the art neon screwdriver..

 

[malcolmsanford]

Isolate and LOCK OFF THE BREAKER at the dis board and find the corrisponding neutral and remove from the neutral bar then perform a safe isolation test. Once confirmation that safe isolation has been achieved then and only then is it safe to work on. Serves him bloody right.........

Glenn though in practice a good theory not really practical doing that. The only 100% safe way to isolate is by turning the whole board off from the double pole switch mate.

As this is the real world it is not always done, so guys do the next best thing which is isolate the circuit and then test the FCU, which by the sounds of things the gas fitter didn't do.

If you start as you suggest, taking off the front CU cover, and digging about in a live CU looking for a neutral, you are really putting yourself in more harm than doing good. Also there is no guarantee that the 3rd MCB along, the neutral is in the 3rd way of the bar. So you could be taking a neutral out of a packed messy board that is still under a load.

 

[tical]

Thanks Markie sparkie for reminding me of the correct proving dead procedure. Im sure you use your two pole voltage detector and proving unit every time you make safe, well done.