(UK) Earth Fault Loop Impedance - EFLI

[UKEng]

Here in the UK when we calculate a cable size and type of protection to be used we take EFLI and Prospective Short Circuit Current (PSCC) readings at the source.
Usually we measure back into the supply system which is back to a supply transformer through all associated cabling.
The question is, has anyone taken an EFLI and PSCC reading at a fuse board which is fed from a UPS system and if so what values were obtained and were they valid?
With a UPS supply you are not measuring back into a supply transformer but into the electronics of the Inverter. We are using the standard Robin test meters for our readings.
As you will be aware we are trying to ensure our diconnection times for Earth Faults.
If anyone has had experience of this then I would be glad to hear from you.
Many thanks in advance.

 

[frankatlem]

I've not actually made any readings of ELFI or PSCC on a UPS but I would say the readings you get will totally depend on the design of the UPS. I would suggest that the higher the power rating the lower the ELFI (higher the PSCC).

The impedance in this case is going to be down to the characteristics of the design of the UPS, so it's one of those 'who knows?' questions. However, I would suggest that correct fuse sizing of your UPS supported loads will be the best approach for load equipment protection. Hasn't the UPS got it's own internal load protection?

 

[UKEng]

frankatlem,

This was more of a generic question about UPS systems rather than one specific UPS.
UPS units have got their own load protection which leads me on to another problem, discrimination !! I would want the fuse to blow on the faulty circuit and not rely on the UPS protection to go therebye risking losing everything.
One other point is, UPS systems have current limiting facilities and would actually limit the fault current, hence inrease the disconnection time of the fuse.

 

[Graeme38]

Chunky,

As you say, UPS equipment is current limiting. I would speak to the UPS manufacturer to obtain the information.

 

[dpc]

I don't know anything about the UK requirements you mention, but I can you that you're right - UPS system have a very limited capability in providing short circuit current. Many of these systems utilize a static switch to transfer to utility power to clear a fault.

Your UPS supplier should be able to give you short circuit output current, as suggested above. You should also ask for recommendation on the largest circuit breaker or fuse to be used on the output of the UPS that will reliably operate under short circuit conditions.

 

[lyledunn]

Chunky,
As you know EFLI measurement is required to evaluate disconnection times in order to ensure satisfactory shock protection in the various recognised earthing systems (recognised by BS7671). Essentially, the UPS will not fall neatly within such classifications. However, established shock risk criteria can still be used with respect to direct and indirect contact. Measuring EFLI on a UPS connected circuit will only evaluate indirect shock protection with respect to the mains. When the main supply is unavailable and the UPS source is used then EFLI is irrelevant. Nevertheless, as required by Part 7 BS7671 the system must be tested and indirect shock protection measures confirmed (as well as PSCC overload etc).
So how do you do it? Well, I agree with your post, these systems are often ignored in the testing procedure resulting in breaches of BS7671. There is often a distict lack of information provided by the designer to help assess compliance. This is further exacerbated by little info from the manufacturer. Having had the same problems as yourself I wrote to the various manufacturers and other authoratative bodies. I have an interesting response from NICEIC with respect to the self same question but just cant put my hand on it at the moment. Will post later. Regards,

Lyledunn

 

[UKEng]

All,

Many thanks for your replies. Lyledunn, I look forward to reading your response from NICEIC.

Chunky

 

[lyledunn]

Chunky,
Sorry to dissapoint you, I just cannot find that communication - if you saw this office of mine you would know why! Anyway, my concern for writing to the NICEIC in the first place was not unlike yours. A UPS served a communication centre in Ambulance Control and was upstream of a 1p dist board which in turn served a number of BS1363 socket outlets on a number of separate circuits for the communication apparatus. My concern was really that no discrimination existed since the current limiting aspect of the UPS would take precedence. As it turned out they had to modify their system.
I was also concerned about shock protection measures for the UPS served circuits on mains failure. Afterall, even with current limiting properties, voltages are in excess of those indicated as detrimental to humans in IEC 60479.
The crucial thing is to establish that the shock protection measures listed in BS7671 are met as a minimum. Doing this is difficult enough without also establishing that discrimination is effective!
Actually, Chunky I think I will throw this out as a post just kor interest sake.

Regards,

Lyledunn

 

[Graeme38]

Lyledunn,

To provide shock protection on the supply side of the UPS I always use RCD's. Whilst this tends to negate the principle of the UPS, if you correctly design the circuits for earth leakage in equipment i.e. limit the earth leakage to within say 10mA via, dedicated circuits I have not had any problems. You must obviously obtain details of earth leakage from equipment from the suppliers. My experience has tended to be with large communications rooms which are generally occupied 24 hours a day.

 

[jbartos]

Suggestion: Visit

http://www.avointl.com/documents/ET_Fall_2001.pdf

for more info on PSCC
NICEIC does not appear to have anything readily available regarding ELFI and PSCC