Guess the fault... friday quiz

[Adam_151]

first of all, can I ask that anyone who skim reads this, and immediatly goes "I know what that is" doesn't give the answer away immediatly

Anyway, this morning I went to do a domestic PIR, background is that homeowner had experienced some issues with RCD protection for the sockets tripping and had been recommended a PIR to see what issues lie within. In the short term, it seems that someone has replaced the RCBO with an MCB and taped two bits of tape in a cross over a socket behind the TV and told the homeowner not to use it.. ho hum.

So even before I get the test kit out, I go and inspect the socket in question, I unscrew it from the wall and notice there is quite a bit of corrosion in the box, but other than that, nothing looks too much amiss (apart from a mising gromet), I go to screw the socket back for the time being, as I do so I feel a tingling in my hand... I immediatly pull the socket forward again and try and work out what i've touched, not finding anything I go to push it back again, eyeballing what I'm doing closely to see if I can work out what I felt. I'm supprised to notice that as I do so, the screw seems to draw small arcs as it touches the backbox lugs. I'm now sure that there is something seriously wrong here, and I bang the 3 neon socket tester in for a clue... and it indicates Live/Neutral reversal.

Anyone like to take a guess?

 

[sparkyspike]

Is the neutral not connected in the CU properly? It could be that on removing the RCBO the neutral was put in the wrong terminal.

 

[Adam_151]

Is the neutral not connected in the CU properly? It could be that on removing the RCBO the neutral was put in the wrong terminal.

No, the board is not of a split load type (single incommer controlled), and the Neutrals were correctly connected to the neutral bar

 

[ccrowe]

You mention corrosion. Was there any moisture present? A water leak or spillage shorting the socket perhaps?

 

[Adam_151]

You mention corrosion. Was there any moisture present? A water leak or spillage shorting the socket perhaps?

There was a damp problem on the lower part of the wall causing the corrosion, the damp was not a cause of the fault, however I expect that it probably helped some of the symtoms I described manifest themselves in the way that they did

 

[BS3036]

I'm thinking two separate faults.

I assume the backbox wasn't earthed, but was a bit earthy because damp.

I don't think the RF earth was connected correctly, and I wonder if someone had also wired an appliance plug incorrectly, say swapped N and E?

Long shot though - two faults.

 

[ccrowe]

Missing grommet putting the cable insulation in direct contact with the corrosion? Had the corrosion eaten through the insulation?

 

[Adam_151]

BS3036 is very very close

There did not seem to be any damage to the cable around the missing grommet

More info:

It was a spur, well I say a spur because there was only one cable at it, but there was no continuity on the ring (or what was supposed to be a ring), so either a spur, or and end/branch of a radial that is fused at the wrong level, or however you want to look at it

It was the only socket affected, and there was nothing plugged into it that the time

 

[BS3036]

Oh well, last go. Clearly the earth has moved to being pretty close to live.

So it has fallen out at the next socket along, and is touching a live.

or

It has fallen out at the next socket along, and there is a screw through the cable, joining live to earth.

or

There is a screw through the cable which has cleverly cut the earth, connected this socket's end of the earth cable to live and managed not to still be touching the other half of the broken earth wire.

Sound like the wiring could use some attention whatever it was!

 

[Adam_151]

Well I didn't actually find the fault itself (I was there to inspect, not to fault find), but I discovered the nature of it, and it was as you allude to, that the CPC was broken and that downstream of the break it has been sorted to live

The homeowner tells me that the floor was changed from a timber floor to a poured concreate one about 2 decades ago, and thinks the cable was just set into the concrete, and thought perhaps that over time the concrete had eaten into the cable... I suggested that perhaps a fixing through the cable at some point would be more likely

Another possibility is at some point something caused a live earth fault, and someone kept replacing the fuse/reseting the breaker until it cleared, until the CPC (probably much narrowed) around the point of the fault (probably a fixing) blew clear

It went down on the report as a code one, and I removed the socket, connector blocked all three conductors, put a blank plate on and issued a danger notice.

 

[BS3036]

 

[sparkypete1]

deffo a crossed line some where. did you do the 3 stage ringmain test, ie r1 rn r2 then ri/r2 crossover, that would have deternined if you had a "ring" or a cross over. did you then meggar the cables, domestic voltage at 230v so a reading of 500 megg ohms would indicate perfection all the way down to 2 megs, under that it would be a fail, also you can take the defective link out and reclass the ringmain as a radial, also changing the rcbo to 20amps rether than the 32amps. if the house is over 20 years old maybe a rewire is in order. a full periodic check would solve that.

 

[RF Lighting]

a full periodic check would solve that.

this morning I went to do a domestic PIR

 

[Adam_151]

deffo a crossed line some where. did you do the 3 stage ringmain test, ie r1 rn r2 then ri/r2 crossover

No I only got as far as doing the end to ends, realising there was no ring final continuity and marking it down as requiring urgent further investigation

that would have deternined if you had a "ring" or a cross over

I determined that I had no ring at all

. did you then meggar the cables, domestic voltage at 230v so a reading of 500 megg ohms would indicate perfection all the way down to 2 megs, under that it would be a fail

Yes, I found low IR on the circuit as well... that also got marked down as requiring urgent further investigation

also you can take the defective link out and reclass the ringmain as a radial, also changing the rcbo to 20amps rether than the 32amps.

I wasn't there to fix faults, anything beyond making safe any immediatly dangerous situations I find, is out of the remit

if the house is over 20 years old maybe a rewire is in order. a full periodic check would solve that.

My perioidic recommended a rewire of the socket outout circuit, the rest of the installation required various other remedial work

 

[sparkypete1]

hi op thats the difference, the housing company i work for states when i find any faults while testing I have to put right said fault, even if it means a full rewire.