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Piratepete

Hi Guys

Help! The 6 amp MCB for a downstairs lighting circuit keeps tripping, sometimes instantly, sometimes slowly. An IR check live to earth gives >200 Meg on the 250 Volt range (I don't want to blow up the dimmers!) when all light switches are OFF! This reduces to near short circuit when you put ANY switch in the ON position. Why?

Lights, switches and dimmers in all the rooms work as they should.

Domestic installation, about 10 years old. TNCS, 16th Edition box. Very difficult to fault find because no pendants with a loop circuit. Rooms either have wall lights or downlights. Dimmer switches in all rooms. All switches are metal plate. I've opened up most of the switches to check for pressed wires. There are no outside lights, bell transformers or smoke alarms on the circuit. Isolating an extraction fan makes no difference.

I'm hoping the solution is obvious, but it was a 4pm job ad I was already brain dead1

Cheers
Pete
 
Hi Guys

Help! The 6 amp MCB for a downstairs lighting circuit keeps tripping, sometimes instantly, sometimes slowly. An IR check live to earth gives >200 Meg on the 250 Volt range (I don't want to blow up the dimmers!) when all light switches are OFF! This reduces to near short circuit when you put ANY switch in the ON position. Why?

Lights, switches and dimmers in all the rooms work as they should.

Domestic installation, about 10 years old. TNCS, 16th Edition box. Very difficult to fault find because no pendants with a loop circuit. Rooms either have wall lights or downlights. Dimmer switches in all rooms. All switches are metal plate. I've opened up most of the switches to check for pressed wires. There are no outside lights, bell transformers or smoke alarms on the circuit. Isolating an extraction fan makes no difference.

I'm hoping the solution is obvious, but it was a 4pm job ad I was already brain dead1

Cheers
Pete

Think about it - what does the switch do?
 
Maybe the answer is in your last statement. Just go back with a fresh mind and approach and you might find it easily.
 
TNCS the C means combined once you switch on the light the live is connected to neutral via the lamp and the neutral is connected to earth via the combining link in the head. Hence the low resistance.
 
TNCS the C means combined once you switch on the light the live is connected to neutral via the lamp and the neutral is connected to earth via the combining link in the head. Hence the low resistance.

Oh, silly me! I'll disconnect the neutral!

Thanks
 
Something is obviously making it trip. Disconnect the neutral, turn off all switches, if clear progressively turn the switches on and see if a fault occurs.
 
From what you have posted as it is on a mcb and is clear when dimmers are turned off it is one of the switch wires or at one of the fittings /between fittings
 
I would take the "sometimes instantly" as a clue as to where the fault lies,as whatever causes it to trip instantly,may be the problem
 
Is the circuit RCD protected? Possibly if the install is only 10 yrs old. If so it's not going to be an earth fault but a L-N fault. These tend to be harder to trace because any lamp/load left in circuit will give a reading, therefore it is necessary to disconnect all loads to trace the fault. Often with such faults the culprit will be a light fitting rather than the wiring.
 
Hi Guys

Help! The 6 amp MCB for a downstairs lighting circuit keeps tripping, sometimes instantly, sometimes slowly. An IR check live to earth gives >200 Meg on the 250 Volt range (I don't want to blow up the dimmers!) when all light switches are OFF! This reduces to near short circuit when you put ANY switch in the ON position. Why?

Lights, switches and dimmers in all the rooms work as they should.

Domestic installation, about 10 years old. TNCS, 16th Edition box. Very difficult to fault find because no pendants with a loop circuit. Rooms either have wall lights or downlights. Dimmer switches in all rooms. All switches are metal plate. I've opened up most of the switches to check for pressed wires. There are no outside lights, bell transformers or smoke alarms on the circuit. Isolating an extraction fan makes no difference.

I'm hoping the solution is obvious, but it was a 4pm job ad I was already brain dead1

Cheers
Pete

If you are testing between Line & cpc then the switches being off or on shouldn't make any difference to the readings. A short circuit is between line & neutral! So are you actually testing between line & neutral, then operating the switches?

Jay
 
The need for "back to basics" testing is apparent. All other loads,etc,will need to be isolated or disconnected.

A degree of patience is also required,as even such a "heavy" fault,will not always be obvious.

I have regularly encountered faults heavy enough to trip 32A mcb's intermittently,which did NOT show up on dedicated IR tests.

Thermal,and many other influences,can alter readings from acceptable,to capable of incurring the operation of an OCPD...and back.

This is why, this job is so much fun...o_O
 

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