Guess the Fault???

[Darkwood]

Thought id start a thread for posting those unusual faults that have you scratting your head for age, feel free to post one for a guessing game but here goes with mine....

Call out to a work yard who have a metal container cabin decked out with a few sockets and lights, the problem was nothing worked and they had checked mcb's and rcd's.

So just to clue you up and no i didnt wire it but this is how i found it... 180amp 3ph generator feeding a dist' board DB1 in an adjacent cabin- off this DB1 it also fed a small consumer unit in same cabin DB2, it was a front end rcd on this board and one of its outgoing circuits fed the steel container dist' board DB3 that was having problems.

First i check DB2 rcd and outgoing mcb were on --- yes
Then i opened DB3 and checked for voltage --- L-N Ov / l-E 230v / N-E 230v (note single phase board)
Next i checked the power in DB2 ... front end rcd L/N input 230v --- rcd output L/N 0v
rcd output to E was L/E 230v & N/E 230v
This rcd was in the on position what was my fault and why was i getting 230v N/E and L/E when i had switched off the mcb powering DB3 (note mcb was working fine)

Hope you took all that in lol

I just add for more confusion ... DB2 and DB3 had front end rcd's 30mA and the sockets off DB3 were RCD sockets....YEP!!! crazy :6:

 

[Darkwood]

If i turned off the mcb feeding the DB3 i lost the L/E 230v as expected in DB3 but retained 230v N/E Why?

 

[snowhead]

Neutral failed between DB2 and 3

 

[plugsandsparks]

Next i checked the power in DB2 ... front end rcd L/N input 230v --- rcd output L/N 0v

Got stuck at this bit. If you had 0V across RCD output in DB2, were not all circuits off DB2 affected as well as DB3

 

[Darkwood]

it was the only circuit on DB2 but you spotted missing voltage so why have i got the 230v N-E then if i have no outgoing feed.... took a while to figure this out even checked the gen's N/E link was connected to which is was.

 

[Darkwood]

No earth was fully functional and fine no issues snowhead

 

[Geordie Spark]

The annoying fault in my case was being born in England, speaking English as my first language, having white skin and a strong work ethic.

 

[plugsandsparks]

RCD in DB2 has blown on outgoing side putting 230V on neutral outgoing ?

 

[cogsey133]

Told you before Geordie you may be of scottish descent so come on ower the dyke neebs. ya ken ya wanna

 

[Darkwood]

You close Plugs but il let you know now and was a bugger to work out but elimination is the beast and thus the most unlikely becomes the likely.

DB2 -DP fron end RCB or RCCB if you like had tripped and when reset had mechanically latched but not closed Neutral fully and on top of this the test button was pressed by customer trying to get power on now in doing so it created a high impedance circuit using all the outgoing devices to create a false N as its tapped of the outgoing side of the RCD and with no true N available it used the all the neutral outgoings to create enough current to weld the test button contact together...... so i had no true neutral and a 230v potential to earth from any outgoing N thus turning the DB3 mcb off didn't isolate the neutral going to DB3 and through connected loads.

 

[plugsandsparks]

Strewth, it reminded me of an EICR i was doing. A number of snap cabins with SP Dbs with front end RCDs. After pressing the test button, bloody thing conked out so no power and shift break in 40 mins. EEK - needless to say i legged it.

 

[telectrix]

no neutral continuity through RCD. buggered RCD.

 

[Darkwood]

Its probably a 1 in a million due to the mechanism not latching correctly which could have stopped it tripping with the test button as well as either mechanically jamming the test in or welding it closed which i see less likely.... was a three way failure of the device but allowed feed through initially then High Impedance fault back up Neutral due to jammed test mechanism.
Nightmare to work out especially with 3 sets of 30mA rcds in a line, it just didn't make sense, i even had them shut genny down so i could verify the N/E link was functional and not loose.

Always a school day :whatchutalkingabout

 

[Darkwood]

no neutral continuity through RCD. buggered RCD.

Answered in post 10 tel and your correct but not fully you wouldn't get 230v N/E in D.B3 if the mcb feeding it was switched off liked i found, as mentioned in my OP this is for the serious head scratches i would have quickly realised just a broken N and not thought it worthy posting.

 

[i=p/u]

starting with your r1 and r2 wouldnt of helped me this time...

- - - Updated - - -

but could of lent them an extension lead and dispossed of 3rd board

 

[Darkwood]

I missed explaining the full test sequence to get to the relevent by this point id already disconnected the suspect feed cable and did all relevent tests on it and it really was the fact that turning the supply mcb off still left 230v N/E at DB3..... was quite a relief when my next breakdown was a drive or encoder fault

 

[i=p/u]

if that had hgappened to me i would of thought polarity was wrong

- - - Updated - - -

id be turning in circles and scrathchin the scalp

 

[Darkwood]

if that had hgappened to me i would of thought polarity was wrong

- - - Updated - - -

id be turning in circles and scrathchin the scalp

All was checked 3ph genny up to DB2 and with DB3 breaker on the L1 became live 230v to earth and 0v L/N in DB3 so couldn't be polarity they were both at the same potential on same phase but i doubt il ever see this fault again but it been 20 odd yrs in the waiting

 

[Geordie Spark]

Told you before Geordie you may be of scottish descent so come on ower the dyke neebs. ya ken ya wanna

I have to say that when I was working in the Rock & Roll business and latterly in Exhibition Contracting I did enjoy working "over the border". It was guaranteed to be well looked after & to have a great time. One of my fondest memories was working with the crew at the Glasgow Theatre Royal & the party that followed. :17::54:

Great lads the Jocks ...... I'd rather work with them than Mackems any day. :wink_smile: