Poor Insulation Resistance Values with MICC

earthboy · Apr 4, 2010

Problems with a particular lighting circuit in a church run in MICC results in the MCB tripping intermittently. All lamps OK, good earth continuity at all light fittings, but only a L+N to E Inslation Resistance value of 2.23 MegOhm.

I've also been told that when switching on other lighting circuits on the same CCU the same MCB has tripped.

I can't seem to replicate the fault and have tightened all terminals and redressed all cables running into a 4 x 3 gridswitch.

When carrying out L to L Insulation Resistance tested between the various circuits on the CCU I have had very low readings (ranging from 0 MegOhm to 0.85 MegOhm). Some of the other circuits are visibly no where near to the lighting circuit in question (i.e. in different locations of the church and running in separate cable runs). By removing the lighting circuit neutral conductor from the neutral rail in the CCU, the values increase to 3.5 MegOhm.

Not experienced enough in MICC cables and possible induced voltages or back voltages?

Finally, another intermittent 'fault' is that when turning on the lighting circuit in question, there is sometimes a 'humming' noise coming from the CCU. When applying forward pressure to the furthest right MCB, the humming ceases.

Can anyone help explain what maybe happening?

Notsosmart · Apr 4, 2010

Your last comment possibly indicates that the Mcb has not been connected to the busbar correctly (possibly when installed the clamp missed the busbar tab so when you try the screw for tightness it appears to be ok)

MICC does suffer from inductance (and capacitance) so this is a possible problem but only within the circuit itself the outer copper core does not allow inductance from external influences.
Also the mineral (magnesium oxide) is Hydroscopic so it sucks up moisture like a sponge, therefore if one of the pots isn't made off properly or there is a pinhole in the outer sheath then this is the likely cause; does the situation rectify itself in the summer or has it only just manifested? ~ unfortunately in a church the temperature in the summer isn't always hot enough to dry out the moisture.

Guest123 · Apr 4, 2010

An MCB will operate on an overload or a short circuit. Either as Notsosmart said there is a faulty pot allowing the magnesium to suck the moisture out of the air and just doing enough to short between L+N or L+E or there is an overload issue....I'm leaning towards it because 1. you say the breaker is humming when in use and 2. you say it happens when additional lights are switched on.

Have you tried putting a clamp meter on the outgoing line to see what the circuit is drawing??? Alternatively it may be inrush current....what type of light fitting is on the circuit and what type of MCB...B,C etc.

A humming MCB can mean other things too.

DurhamSparky · Apr 4, 2010

Lenny said:
A humming MCB can mean other things too.

what like it wants to break free and set the room alight with its dancing grooves!!??

Deleted member 9648 · Apr 4, 2010

I've had one or two old 3871 mcbs start tripping intermittently for no obvious reason and replacing the mcb has solved it, as Lenny says though first port of call if other tests are satisfactory is a load check with a clamp meter.If that suggests the mcb is not overloaded try replacing it (same type and rating). Another possible cause is adjacent fully loaded mcb's in a packed DB generating enough heat to cause nuisance tripping,sometimes moving a circuit to a spare way with some air space can help.

oldtimer · Apr 4, 2010

You sometimes find with micc that it could be a damaged outer sheath or pot and the internal insulation can get damp hence the low insulation best thing to do is split the circuit up to pinpoint the faulty segmant also when meggering ALWAYS discharge the end to earth if not the prepare for a good wack as they say

Guest123 · Apr 5, 2010

DurhamSparky said:
what like it wants to break free and set the room alight with its dancing grooves!!??

Just like Har Mar Superstar.......

pevvers · Apr 5, 2010

My money is on a faulty MCB, as an insulation resistance that high is never going to cause it to trip... It wouldn't be satisfactory in the regs if it would cause MCB's to trip, and sometimes you look into things too deeply MICC not installed properly is susceptible to low IR's. I've ripped apart a whole external lighting circuit before trying to find what was tripping it, only to realise at the end it was the MCB being faulty...