Earth fault/leakage current tracing

[sparkydude]

Hi all , we have a situation on one of our current jobs with a substantial earth leakage , varies from 50A to 3300A the other day for a millisecond LoL, All cables are swa using the armour as cpc, in this scenario obviously we have some hefty cores to deal with and all are three phase. I doubt the meter above would be upto the job as the cables are upto 300mm 4 cores Anyone got a recommendation as to how we would go about tracking this down , bearing in mind we have approx 36 cables to test I was thinking we would need a meter with sepperate clamp cts to go round the cables to monitor it .

Any help appreciated


Nick

 

[telectrix]

think you'll have to go down the route of isolating and IR testing here.

 

[Chr!s]

Whats the supply type, PME?

What sort of equipment is connected?

 

[sparkydude]

Supply is TNCS supplied via two new 11kv transformers sited outside the switchroom. Equipment conected is general submains feeding subpanels and several operating theatres , and a couple of MRI scanners.

We need preferably a non intrusive method of pinpointing the circuit/s with issues as being a hospital isolating parts for any kind of testing is such a nightmare its not funny,


Nick

 

[IQ Electrical]

What did you use to measure the fault duration?

 

[sparkydude]

The fault current is being constantly monitored on the main panels SEPAM unit , and it is constantly registering between 50-70A now , but as said before it spiked once to 3300A for a millisecond

 

[IQ Electrical]

My first action would be to connect a power quality monitor at the main panel, 1 week duration to capture any 'events' in that period.

I'd make sure that records of the times of use of any specialist equipment (such as MRI etc.) were available to compare with any 'events' highlighted during the monitoring.

Has the designer been consulted regarding an expected level of leakage for the installation?

Is the 50-70 Amp leakage present at all times?

 

[sparkydude]

we have been tasked with carrying out data logging on the outgoing ways but are unsure if this is going to help us in finding this circuit/circuits that have the leakage current on them as they are specifically concerned with overloading of existing circuits .

The 50-70A is a constant fluctuating reading from the SEPAM,

I doubt the designer will be interested, the main panel and submains are 25-30 years old


Nick

 

[IQ Electrical]

we have been tasked with carrying out data logging on the outgoing ways but are unsure if this is going to help us in finding this circuit/circuits that have the leakage current on them as they are specifically concerned with overloading of existing circuits .

The 50-70A is a constant fluctuating reading from the SEPAM,

I doubt the designer will be interested, the main panel and submains are 25-30 years old


Nick

Ah, I assumed this was a newly completed installation.

At some point, you will have to look at isolation of parts of the installation, even on a process of elimination basis, a methodical approach is needed.

I can see an extended 'duty test' of the essential services generator or alternative supply source!

 

[malcolmsanford]

I would go along with IQ but I would think that in a 25 yr old installation under those sorts of loads, your having a gland joint go. Can you also get hold and use of a thermal camera and take some pictures of terminating glands, see if you have any hot spots.

 

[sparkydude]

We are going to be isolating parts of the installation in a systematic fashion beginning in early june as we are jointing and extending all the swa cables form the old 30 year switch panel to the new shiny Schneider Okken panels . They ideally wanted us to try and find this issue before we begin the project but it seems this is not something we can do using non invasive techniques. I may sugest we get a thermal imaging camer and take it from there, they can only say no LoL.

Nick

 

[Des 56]

Replying with thoughts only and no experience of such a problem,so forgive any drivel

I am sure you have considered this but here goes anyway



Maybe a backwards procedure instead of looking at the overall picture

You cant measure the leakage at the main panel because the armourings are all glanded in common
You cant insulation resistance test because of isolation problems
You cant isolate willy nilly because of safety and operating issues

Did the problem occur only after installation of the supply transformers

If you assume that the swa s have not deteriorated and insulation values may be sound
It may be long drawn out,but may get you where you want
Working on the extremities of the installation at a final circuit level with your earth leakage clampmeter may well be one way of highlighting the issue


Beginning at the distribution boards and getting a total for each board
Highlighting those with high earth leakage and then pinpointing to which circuits the leakage belongs, and then looking at whats used on those circuits, any unintended leakage may be highlighted
Perhaps the leakage is simply a factor of the equipment used and that the existing installation was not designed with earth leakage as a functional property at that time




I just read that leakage is not permitted to exceed more than 1 ten thousandth of maximum demand
Does yours

 

[sparkydude]

Thanks Des, your idea is what we were thinking of doing, identifying from the field end , any high results and then taking it from there. As for the maximum earth leakage permitted , not sure but i am confident our supply is less than 500,000A LoL


Thanks again all ,

Nick

 

[Engineer54]

What sort of input are you receiving from the Hospital Engineering Manager?? Was this situation present before any of the work your company has undertaken?? Are any of the main DB breakers tripping out on earth fault at the main switchboard or at the load end Breaker??

Just trying to get a little more information on your situation, as your switchboard is supplying the complete electric needs of the hospital, and i would imagine the Stand-by Gen-Set will also supply a good section of the switchboard. This will further complicate any fault finding processes...

 

[Engineer54]

You will probably find that the biggest leakage areas to be from MRI loads, and from X-Ray departments on the medical side of things and from main A/C plant and General Service Buildings /Areas...

As been suggested, a thorough and systematic inspection with a good thermal imaging device could well identify any heat source faults such as cable end terminations/joints etc...

You say that your supply is TNC-S, are you sure about this?? Often the neutral point and earthing system are made at the main switchboard and not at the distribution Transformer. This is for the ease of locating protection relay CT's, but will often confuse those that are not familiar with this arrangement. It's not usual to see TNC-S supplying hospitals especially when the hospital is being supplied with it's own 11KV/400V Transformers....

 

[sparkydude]

Basically this problem must have been present in the past , but there was no SEPAM unit on the old panel. Sorry i meant TNS suppy, Doh.
We have won the contract to do the swap over from new to old panels and re-test/certify the newly extended sub mains leaving the new panels. as all the DB's use HRC fuses as their feed, nothing has ever highlighted it previously . As i said earlier and i am glad to have had it confirmed by you the MRI scanner is a likely candidate so we are most definiteley going to be targeting that as one of the first things.
This is not the only supply to the hospital but one of 4, all the other supplies are on the new parts of the hospital we completed last year, their SEPAM relays are reading 0A , its just this old part of the hospital that is causing issues.

Nick

 

[Engineer54]

So there are 4 other incoming supplies to the hospital, ...I take it then that the OP theatres, X-Ray, MRI facilities are centred on the older part of the hospital then, ie this new TX / switchboard your presently working on??

Are you not getting any input from the Hospital's Engineering Manager in this problem?? He would (should) have an in depth working knowledge of his hospital, and be able to give your company all necessary information to help you isolate/locate possible problem areas...

Did the old fuse switch switchboard not incorporate any protective relays that tripped out the fuse switches on identifying faults in the system ?? Even the older manual fuse switches had solenoid trip function, operated by protection relays!!

Just a thought here, has anyone checked the new switchboards control wiring out, this reading your getting on the SEPAM unit could be from a miss-wiring fault. I can assure you it's not an uncommon occurance. Normally these faults would have shown up during the the clients factory visit for inspection and testing, prior to being delivered. But even then, as all the individual panels are then separated for delivery, reconnection of the control side can get miss-connected, it's happened more than once on my projects... It worth a check anyway....

 

[sparkydude]

Yes thats right, the majority of the new parts have the theatres etc in them but there are the older parts still containing certain departments still remaining . The hospitals engineering manager is not a great deal of help they just trust the reading on the SEPAM and are trusting us to sort it out, I will mention it to the project manager tomorrow and get him to have the SEPAM connections checked to ensure they are correct. None of the old panel had any kind of solenoid trips on them just switchfuses and ACBs on the generator connections.

Thanks again for your help


Nick

 

[Engineer54]

That's the second time i've been told that a Hospital Engineering Manager isn't up to his position!!! Things must really be going down hill in the hospital engineering field is all i can say!!!

That old switchboard must have been cheap as chips when it was installed ...lol!!! What about the bus coupler, was that not automatic either, if there was one that is ...lol!!! Or was the generator sized for the whole of one side of the switchboard containing the essential loads?? ...I hope the new MG LV switchboard is a far cry from that old switchboard set-up, but then being MG it certainly should be!! lol!!!

 

[malcolmsanford]

That's the second time i've been told that a Hospital Engineering Manager isn't up to his position!!! Things must really be going down hill in the hospital engineering field is all i can say!!!

Over the years I have worked with 3 hospital engineers and all of them were ex chief Engineers from the Navy, 2 liked the sauce and the 3rd was a bit of a stickler, but all 3 were top guys, but knew absolutely nothing about electrical engineering.

Strange how even at the top of the food chain like site forman and Clerk of works, you very rarely find an Electrical Engineer running a project, I suppose the electric side is too complicated lol.

Top posts E54,I still go down the line that there are terminations and joints breaking down, would be nice to get a thermal camera on these to just see.