Finding a very awkward fault

[mattg4321]

Hi guys (and girls)

Have a job for a long term customer, finding a pretty awkward fault. I'm almost to the point where I'm a bit stumped. Generally speaking IR testing, RCD testing and experience hasn't let me down in the past and I've found plenty of faults other 'electricians' haven't.

Symptoms are:

Loud bang, possibly from kitchen area, happens around once every 3-4 weeks. Downstairs RFC RCBO trips out. This covers all sockets downstairs, bar 3. It has apparently happened twice overnight, when no appliances are being used - customer has come downstairs to find circuit off. This has been ongoing for a few months now. The consumer unit is at the other end of the house to the kitchen.

Testing so far:

All loads disconnected Line-Earth, Line-Neutral and Neutral-Earth all reading over 70Mohm.
All loads connected L&N connected together tested to Earth - 5.5Mohm
I thought I found it when I found an old fridge with a very low resistance line to earth. Sadly not. The fridge has been replaced.
Nearly new Hager consumer unit (fitted last year) RCBO ramp tested. All ok.

Conclusion:

The customer is a liar! Highly unlikely in this case.

or

Appliance is causing the issue. What would cause this at any time of the day randomly when nothing is operating though? How to confirm this without a scattergun approach of renewing appliances/plugging them in to different circuits?

or

Something is happening to the fixed wiring once a month to case a 'bang' and a fault. Seems highly unlikely to me with the IR readings I'm getting.

Anybody got anything to add? Hopefully I'm missing something blindingly obvious here! One downside of fitting RCBO boards is I don't actually know whether or not I'm probably looking for a L-N or a L-E fault, which is slightly unhelpful.

Cheers

 

[static zap]

As Midwest mentions , a Day date
(add a timer to record loss ? )
... Any subsidence cracks in property ?
Settling extensions added ...
Get a magnet out for RSJs !
Thinking track with Hot / cold / damp weather .

 

[sparkdog]

I have had a back box going Bang but the insulation was still reading above 700 Megohms. Also had cable buried in kitchen wall going Bang but reading OK.Only fix was to locate the Bang and replace the cable.

 

[PEG]

The truth is,on occasion,bar hitting it with Lucien's Ground detonated EMP device ,a fault of this nature,can only be found by touch,smell or sight.

You can have situations where a hidden fault,can pop a 32A MCB every 5 minutes or once every 5 months,and any normal IR test will read fine.

There can be many reasons why this occurs...change of temp...vibration...or just why two coins land heads up...

The reasons why one man can find the fault,may have nothing to do with knowledge or kit. It could be persistence,time or even luck.

If you want a career with guarantees...be an undertaker

 

[marconi]

IR tests carried out at 500V with CPC's connected to earth bar.

Will try at 1000v on my return.

Re: SJD #16

When you next visit, if you don't discover the fault then, you want to leave the home and this FRC in a state so that if the fault occurs in your absence it can be narrowed down to a shorter section. So be prepared to section the FRC as SJD suggested into two radials or even better 3 radials if there is the possibility of running a temporary length of 2.5mm2 T&E to power a 'middle' ring section. You'll need some way of providing temporary protection for each section which you could think about and plan for beforehand, and some way of labelling their breaker.

 

[Andy78]

I had this exact fault once. Banging noises reported from kitchen, every few weeks, blew the 30A fuse wire every time. They didn't actually tell me about the loud banging until I'd started investigating as they thought the noise was coming from the fridge. The old lady customer had been convinced by her son that modern fridges made these noises when defrosting. This was also despite the fridge being about 25 years old.
Turned out to be a fault that had probably been introduced when the kitchen was extended some 15 years previous. The builders had spurred off the ring and wedged the spur cable betwixt joist and brick, damaging it in the process. It had kept blowing and creating further damage to the point where about 2 metres of the cable under the floor was charred and exposed. Eventually calling her son to replace the fuse wire got a bit too frequent.

Can't seem to upload a pic.

 

[Highresdis]

As an easy next step, I'd suggest asking the customer to temporarily connect the appliances to other circuits with extension cables if possible, to see if the fault follows the appliances or not.

 

[marconi]

When you re-do the IR tests I suggest you do it in two phases. The first time with all socket and FCU switches in the off position and the second with them all in the on position. The FRC may have a faulty or contaminated socket or FCU.

 

[marconi]

Some further things for you to consider:

1. Look at the 13A plugs using the FRC to see if there is any surface tracking between the L pin and the N or E pin. Open them up too. Pay particular attention to moulded-on plugs with the fuse carrier between the L and N pins.

2. Have the paper 'how to wire' instructions been removed from plugs?

3. Ask the owner if any plugs in use have ever been dropped in water or other fluid. Or may be wetted by a leak/overflow from a washing machine or dish washer.

 

[Marvo]

......1 VDE approved insulated ferret
duct tape

The tape is just for strapping the phone on. Do not try to insulate a regular ferret with tape, use a proper live-working ferret.

*tears*

If you're ever arrested in posession of a ferret, an iphone and a roll of duct tape you might want to avoid using this line in a court of law as your defence.

 

[CACJL]

Hi guys (and girls)

Have a job for a long term customer, finding a pretty awkward fault. I'm almost to the point where I'm a bit stumped. Generally speaking IR testing, RCD testing and experience hasn't let me down in the past and I've found plenty of faults other 'electricians' haven't.

Symptoms are:

Loud bang, possibly from kitchen area, happens around once every 3-4 weeks. Downstairs RFC RCBO trips out. This covers all sockets downstairs, bar 3. It has apparently happened twice overnight, when no appliances are being used - customer has come downstairs to find circuit off. This has been ongoing for a few months now. The consumer unit is at the other end of the house to the kitchen.

Testing so far:

All loads disconnected Line-Earth, Line-Neutral and Neutral-Earth all reading over 70Mohm.
All loads connected L&N connected together tested to Earth - 5.5Mohm
I thought I found it when I found an old fridge with a very low resistance line to earth. Sadly not. The fridge has been replaced.
Nearly new Hager consumer unit (fitted last year) RCBO ramp tested. All ok.

Conclusion:

The customer is a liar! Highly unlikely in this case.

or

Appliance is causing the issue. What would cause this at any time of the day randomly when nothing is operating though? How to confirm this without a scattergun approach of renewing appliances/plugging them in to different circuits?

or

Something is happening to the fixed wiring once a month to case a 'bang' and a fault. Seems highly unlikely to me with the IR readings I'm getting.

Anybody got anything to add? Hopefully I'm missing something blindingly obvious here! One downside of fitting RCBO boards is I don't actually know whether or not I'm probably looking for a L-N or a L-E fault, which is slightly unhelpful.

Cheers

I had a similar fault about 20 years ago, it was caused by a ring main cable passing through UR cavity wall insulation which had overheated, it went bang about one a day.

 

[dmxtothemax]

5.5Meg seems a little lower than normal ?

 

[ipf]

5.5Meg seems a little lower than normal ?

It might have gone up a bit in 20 months..

 

[DPG]

Dmx does like resurrecting these old threads!

 

[SparkyAndGeorge]

Did you ever find the source of the fault?

 

[freddo]

Probably rewired by now!
If you don't mind the possibility of starting a fire you could hook up metal halide control gear to the circuit, the ignitor will cause the problem area to flash over and the ballast will limit the current preventing fuse blowing. Then just listen out for the arcing sound...