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....