Another Strange Fault

[securespark]

Went to a property today, the complaint was intermittent tripping of two lighting circuit mcb's with no obvious pattern.

The breaker was going with a heck of a bang, but there was no obvious noises out in the field, so to speak. There is no RCD on the circuit. The supply is single phase.

At exactly the same time, another breaker was popping. This other breaker was situated at the other end of the board, so I cannot blame thermal influences. I have also ruled out interlinking as there only needs to be one breaker off to isolate the circuit.

The fittings were removed (a mix of SELV d/l's & mains pendant fittings) and any joints and transformers examined. The dimmer module was taken OOC after a voltage check.

Here are the results. I have not had time (again!) to strip the circuit down & check IR yet. Monday's job.....:


L/N = 12V

Sw/N = 12V

Sw/E = Varies with dimmer

L/E = 400V.

I have three things that don't quite add up:

(i) the fact that two breakers pop.

(ii) The varying voltage.

(iii) the 400V reading.

 

[Spark123]

Number of theories,
One breaker feeds the other,
One breaker in phase the other in neutral, (naughty)
One breaker feeds a circuit controlling a contactor, the other breaker feeds the contacts.
Where did you manage to get 400v L-E??
Whats the prize this time?

 

[securespark]

Prize..hmm, well you've already won a well-worn pair of socks. How about a well-worn pair of grundies to go with them?

One breaker feeds another. Good, but I don' think so, unless it does so by a tortuous route. I'll double check that.

Both breakers have red conductors, so unless the wrong colour has been used...you never know, especially on this job!

There are no contactors in circuit - AFAICT.

400V at a ceiling JB between the live loop & earth.

 

[joe-90]

If you were getting 400v in the house more than breakers would pop.

It usually means your meter needs a new battery - I've had the same readings in the past.

 

[securespark]

It does not have batteries!

 

[bernardgreen]

400 volt suggests another phase involved. Any lighting shared with house next door ( intentionally or accidently ) that is on another phase.
Has next door connected to a switch wire in a party wall or up in the loft if there is no separation between lofts.

Or is is a flat and the flat above is on a different phase and its socket circuit has become connected to the lighing circuit you are tasked with.

Or wild shot that once happened to us. Has the incoming neutral become disconnected some where in the network and what should be star point of the supply transformer is now a virtual star point varying with the ratio of loads on the properties on that the disconnected bit of neutral is connected to. Luckily we had a very good earth rod in the banks of a stream so our neutral and therefore earth terminal was close to ground potential. ( Think the fish had warm water though )

Bernard
Sharnbrook

 

[Adam_151]

Always on a friday... someone obviously doesn't want you to knock off early to go to the pub

Anyway, this 400v reading concerns me greatly, either the meter is playing up or there is really 400v there

Did you say the dimmer was off the circuit when you checked the volage?... that rules out that 'confusing' the meter, checked the meter on a known 240v Supply?

If there is really 400v there, I can only see two possibilities both of which have already been given, either another phase coming from somewhere, or a floating star point

What type of earthing system this time?

 

[joe-90]

If you are getting 400v in the lighting circuit then the bus in the consumer unit will also be running at 400v. This means that so too would the power circuits and your tv and video would explode.

It's not happening - your meter is wrong.

 

[joe-90]

It does not have batteries!

No wonder it doesn't work then.

 

[RF Lighting]

So the voltage readings are at one of the flex drops.

Did you take any voltage readings with the main switch turned off, both the at the incommer, and at the 400V flex drop?

 

[plugwash]

firstly DO NOT TOUCH any metalwork that is connected to that circuits

secondly check your meter on a known good mains source

its possible you have a lost neutral and a lost earth and then your earth conductor is shorted to another phase (assuming say a conduit system with multiple phases inside it)

 

[RF Lighting]

It does not have batteries!

No wonder it doesn't work then.

My voltage indicators do not have batteries either.
You do not need batteries to measure voltage.

 

[joe-90]

It does not have batteries!

No wonder it doesn't work then.

My voltage indicators do not have batteries either.
You do not need batteries to measure voltage.

T'was a joke, for heaven's sake.

 

[ricicle]

T'was a joke, for heaven's sake.

We should have realised as the majority of your posts on this forum tend to be comical

 

[securespark]

The supply is TN-S. The house is completely detached.

The voltage reading was at a JB between loop & earth.

The 240 reading was between switch & earth at the same JB, but this voltage varied with the dimmer.

I'm not sure why..

Did you take any voltage readings with the main switch turned off, both the at the incommer, and at the 400V flex drop?

Not yet, but that is what I will do on Monday if I can't get a sensible reading out of it. I will take another too.