Main Bonding Puzzle

[UNG]

Is this a fixing screw into an earthed metal back box that has grazed the insulation on a live cable type fault, wouldn't be the first time it has happened and left people scratching their head for hours

 

[dodger421]

Given the sheer number of bodges and examples of shoddy workmanship from bathroom fitters, kitchen fitters, wet pants and not to mention the DIY electrical works by the previous owner you’re probably spot on.

I suspect when I get the chance to test the pipework just to the external earth the IR reading will improve, but I’ll let you all know when I get around to doing that test.

 

[UNG]

This reminds me of a fire alarm fault I had to deal with around 30 years ago and started my total dislike of FP. This fire alarm developed an earth fault on one zone, an IR test gave very low readings and the continuity readings were over range,
I came up with an idea to use my recently acquired cable tracer and when set up it toned through the fault so into the loft and I followed the tone on the cable to a water tank where the cable was trapped under one of the bearer timbers, tone on one side of the bearer no tone on the other lifted the bearer slightly and the tone disappeared, got the cable from under the bearer did another IR test and got good readings reconnected and tested the alarm and all was good
Since then I have found similar faults in much the same way

 

[LastManOnline]

There’s no mains gas in the village

Thus NO main bonding required

You stated that the water mains is plastic...
Thus NO mains bonding required.

If you still harbours doubts as to whether your incoming services require main bonding you should disconnect the from the MET (by ensuring any cpc, s or bonding clamps etc are removed) and then carry out an IR test between the the incoming service and the MET.

 

[dodger421]

Thus NO main bonding required

Ah if only. As per the essay the incoming LPG supply runs underground in an unidentified fashion before entering the building.

You stated that the water mains is plastic...
Thus NO mains bonding required.

If you still harbours doubts as to whether your incoming services require main bonding you should disconnect the from the MET (by ensuring any cpc, s or bonding clamps etc are removed) and then carry out an IR test between the the incoming service and the MET.

Again as per the essay this was carried out between MET and LPG/water supplies with main bonding and boiler cpc’s disconnected. Continuity was off scale high to both (>2000 ohm), but IR was 0.00 MOhm

@Pretty Mouth what other information would you like?

I’ve still not been able to isolate the supply to test just to the external earth, but I did try repeating the continuity tests with my DMM. I know it’s only a low current test, but it consistently reads 3.4 MOhm. IR testing with the MFT still shows 0.00 MOhm, but this time I also paid attention to the voltage. It’s only reaching 6V before displaying 0.00. MOhm.

 

[Pretty Mouth]

@Pretty Mouth what other information would you like?

Much, much less. I tried to take your post in, but there's too much there fella. IMO you'll get more help, at least from people with tiny minds like me, if you keep it short and relevant.

 

[pc1966]

My guess is some metal pipe work, or some related metal back-box, etc, has a bit of corrosion and is in contact with something damp. When you try with a DMM with its 1V or so test voltage you see very high R but pushing a mA or so from the IR tester's 250V+ capable supply whatever layer has formed gives in and conducts.

 

[LastManOnline]

Is this a fixing screw into an earthed metal back box that has grazed the insulation on a live cable type fault, wouldn't be the first time it has happened and left people scratching their head for hours

I think it may even not be a fault. Most of these type of situations are, nt. If on a continuity test you get a high resistance, but on an IR test you get 0.00(no resistance,. looks like a short but could really be 10,000 ohms) then it's certainly not a direct contact between a live conductor and a cpc. If it was, a continuity test would pick it up. I would now be looking at the possibility of a DP switch with an indicator light left in the on position.These are the kind of situations that wont show on a continuity test, but will on an IR test. Came across this few times having first (and frustratingly) "ripped" the installation apart.

 

[dodger421]

@Pretty Mouth fair one, I did go overboard with the detail.

@pc1966 that makes sense, as above the IR test stops at 6V so whatever it is it’s not making great contact and the insulting material is easily broken down.

 

[LastManOnline]

I came up with an idea to use my recently acquired cable tracer and when set up it toned through the fault so into the loft and I followed the tone on the cable to a water tank where the cable was trapped under one of the bearer timbers, tone on one side of the bearer no tone on the other lifted the bearer slightly and the tone disappeared,

This wins "tip of the month" as far as I am concerned and could well be in contention for "tip of the year" ?