fault today at work 400v live to neutral

[jonney88]

went to work today, on a hospital we’ve been working atand we got there and a phase of works that we’ve have already done I tested itan powered it up last week, the lights were all flickering and one light sw onone circuit was switching another light on another circuit very strange, I tookthe db lid of then checked voltages I got L1-N 400 L2-N 230, L3-N 69v,
even neutral earth 230v, so I immediately turned the full db off faults didn'tdisappear, went down to section board it was an old proteus fitted in the1970's,,we used a spare way on it to feed the new db checked the voltages antthat they were fine,, so I took some loop reading l-n l-e l-e was fine l-n werehigh and unstable to I’m thinking loose neutral I turned off and on thesw a few times put contact grease n the fault cleared all is good,,,but whydid I get 400v down l-n is it due to an unbalanced load trying to balance itsself, and finding an alternative root back to the source via another phase anwhy did the rcbo not trip,,,rcbo only has neutral tail no earth,,,id reallyappreciate people opinions on this took me 3 hours of scratching my head onthis

 

[Lucien Nunes]

Classic symptoms of lost neutral, should only have cost about one second of head-scratching! Yes, the different P-N voltages occur due to the imbalance of load currents, the difference of which would be the neutral current under normal circumstances. Make sure you really did find the fault as lost neutrals can wreak havoc, set things on fire, destroy valuable pieces of kit, cause major insurance claims!

Why would you expect the RCBO to trip? There's nothing specifically to cause overload (although it could happen when things are overrun/damaged) and nothing leaks to earth.

 

[jonney88]

Classic symptoms of lost neutral, should only have cost about one second of head-scratching! Yes, the different P-N voltages occur due to the imbalance of load currents, the difference of which would be the neutral current under normal circumstances. Make sure you really did find the fault as lost neutrals can wreak havoc, set things on fire, destroy valuable pieces of kit, cause major insurance claims!

Why would you expect the RCBO to trip? There's nothing specifically to cause overload (although it could happen when things are overrun/damaged) and nothing leaks to earth.

I think the cause was a sw fuse on the old section panel,, the sw fuse has never been used and is from the 1970's so I think it was just poor contacts,,, once it was turned on and off a few times the fault cleard,, how else can I ensure that was the cause ive never come across the fault before ive been doing the job for 9 years now always learning

 

[TJ Anderson]

Omg, 400v L-N (where art thou neutral), what could it be?? lol

 

[TJ Anderson]

Bull in a china shop fault finding that is. If you are fortunate enough to have it under fault condition in front of you, be methodical and treat with kid gloves. Otherwise you will be guessing.

 

[Marvo]

I think the cause was a sw fuse on the old section panel,, the sw fuse has never been used and is from the 1970's so I think it was just poor contacts,,,

Given the cost implications to the customer with this type of fault I'd suggest removing, stripping and overhauling the switch fuse or replacing it entirely with a new item. A floating neutral could cost tens of thousands in damage to equipment quite easily in a hospital....if not more.... so I wouldn't take the chance that switching it on and off a few times has ensured it won't reoccur.

 

[jonney88]

Given the cost implications to the customer with this type of fault I'd suggest removing, stripping and overhauling the switch fuse or replacing it entirely with a new item. A floating neutral could cost tens of thousands in damage to equipment quite easily in a hospital....if not more.... so I wouldn't take the chance that switching it on and off a few times has ensured it won't reoccur.

why did this have to happen on my job, ive go five other conections to make on to this sction panel,,to remove the sw fuse would be no easy task

 

[jonney88]

if my l-n loop reading are coming back at 0.05 on all phases would u say that's a safe bet the fault has cleared

 

[Marvo]

Lol, I'm not sure what to say really, **** happens. Maybe make a report for the client with a quotation and highlight the risks involved with the high likelyhood of equipment damage and leave the ball in their court plus you cover yourself that way.

 

[TJ Anderson]

if my l-n loop reading are coming back at 0.05 on all phases would u say that's a safe bet the fault has cleared

But you have not definatly prevented the fault coming back!

 

[jonney88]

can I certify the job with this looming

 

[Marvo]

if my l-n loop reading are coming back at 0.05 on all phases would u say that's a safe bet the fault has cleared

A better test would be to monitor the voltdrop across the contacts when it under a decent load or preferrably full load but like I say I'm not usually a gambling man when it comes to hospital equipment

 

[jonney88]

But you have not definatly prevented the fault coming back!

You could say that about anything we do,, I want the advice straight up, I don't want scareing

 

[TJ Anderson]

can I certify the job with this looming

Be straight with them and let them know your findings and implications should it happen again. Fill it in on your EIC (Comments on existing installation). You have then passed the buck.

 

[TJ Anderson]

I am trying to help you mate. Let them know and then sleep easy that they are aware. You did not create the problem, but you found it, so advise and then go from there.