Tripping shower (fault finding)

[Barrington1990]

Hey guys , just been to a fault finding job and discovered what I think may be the problem but the consumer unit may be overloaded , please advise on the following ...

The shower circuit is on a 32a MCB (wrong should be 40a) which I think is causing the tripping of the rcd , my dilemma is that in its current state the consumer unit is loaded up with 3 32a and 1 16a circuits adding up to 116a ... Is this overloaded ? do you apply diversity to consumer unit ? I've done all my tests on shower circuit which is on a 6mm2 cable and all is fine .. am I safe to replace 32a MCB to 40a MCB ? ... Thanks for any help

 

[Pete999]

Hey guys , just been to a fault finding job and discovered what I think may be the problem but the consumer unit may be overloaded , please advise on the following ...

The shower circuit is on a 32a MCB (wrong should be 40a) which I think is causing the tripping of the rcd , my dilemma is that in its current state the consumer unit is loaded up with 3 32a and 1 16a circuits adding up to 116a ... Is this overloaded ? do you apply diversity to consumer unit ? I've done all my tests on shower circuit which is on a 6mm2 cable and all is fine .. am I safe to replace 32a MCB to 40a MCB ? ... Thanks for any help

What brings you to believe that an undersized CB causes the RCD to trip? You don't just add up the CB sizes, apply diversity to the circuits.
Was this not covered on your DI Course?

 

[telectrix]

1. if the MCB for the shower were undersized, then it would trip, not the RCD,
2. rule if thumb with CU load ... add up MCBs and x 0.4.to get an approximation of the actual load (diversity).
3. IR test each circuit for leakage or ramp test RCD with all circuits connected (main switch off). then with circuits dissed from RCD, it there's an appreciable difference, that's your leakage.

 

[Barrington1990]

What brings you to believe that an undersized CB causes the RCD to trip? You don't just add up the CB sizes, apply diversity to the circuits.
Was this not covered on your DI Course?

Because all my tests and checks lead to the under rated CB ... The shower is newly installed so it's not the shower so to speak

 

[telectrix]

Because all my tests and checks lead to the under rated CB ... The shower is newly installed so it's not the shower so to speak

that's like thinking your brakes have seized when the clutch is slipping. you're being led down the garden path.

 

[Pete999]

Because all my tests and checks lead to the under rated CB ... The shower is newly installed so it's not the shower so to speak

If it was proved that the cb was indeed underrated, it would surely be the cb that eventually tripped and NOT the RCD, RCDs trip usually on a fault to E, NOT on an overload as you seem to have suggested. Why could it not be the Shower that's at fault? As Tel suggests IR test the wiring again, after dissing it from the CU first of course, might not be the Shower, could be the wiring that's iffy.

 

[Barrington1990]

Thanks for advice ... I was told the shower had been replaced to rectify the same fault .. but it's still happening .. so I've ruled the shower unit out ... I will report back later on and see if I've resolved the issue

If it was proved that the cb was indeed underrated, it would surely be the cb that eventually tripped and NOT the RCD, RCDs trip usually on a fault to E, NOT on an overload as you seem to have suggested. Why could it not be the Shower that's at fault? As Tel suggests IR test the wiring, dissing it from the CU first of course, might not be the Shower, could be the wiring that's iffy.

He f

 

[Pete999]

Thanks for advice ... I was told the shower had been replaced to rectify the same fault .. but it's still happening .. so I've ruled the shower unit out ... I will report back later on and see if I've resolved the issue

He f

Look forward to your answers

 

[telectrix]

don't forget that the 32\A MCB type B will run at 50A fir about 17 minutes. plenty of time to get wet and clean

 

[Wilko]

Hi - sounds like a N-E low impedance fault somewhere in the installation. Check all circuits for N-E insulation resistance would be my thought.

 

[MJPD29]

Because all my tests and checks lead to the under rated CB ... The shower is newly installed so it's not the shower so to speak

What tests and checks did you carry out? And what results did you get?

 

[telectrix]

I was told the shower had been replaced to rectify the same fault

so that's sombody else batting moths in the dark with a wet hanky.

 

[Barrington1990]

Hi - sounds like a N-E low impedance fault somewhere in the installation. Check all circuits for N-E insulation resistance would be my thought.

Hi
.. turns out it was the upstairs lighting circuit ... Something had chewed its way through the cables above the bathroom.. thanks for the help

 

[James]

Looks like a clear head and some logical fault finding saved the day.
well done and thanks for getting back to us with an update, its nice to know how these things turn out.