Ze and distance from substation

[ringer]

Was shown a PIR by a friend today where the Ze was recorded as 0.01 ohms. Not unexpectedly, the PFC was sky high at 19.9kA. I have never come across a Ze so low before. This is commercial premises, with a single phase TN-S supply. Now the nearest substation I am aware of is 150m away as the crow flies, or 250m away by road, which is more likely to be the route of supply cable takes from the substation to the property. Does a Ze this low seem likely at such a distance?

 

[malcolmsanford]

Was it an actual proper reading.
It is the devils own job normally to get a situation in a commercial premises where you can isolate the entire supply and take a reading.
Some do arrange shutdowns and take a correct reading, some may take a reading when the installation is on just to prove a path and many others make it up as they go along.

 

[TaffyDuck]

0.01/19.9 kA wow, problems with the MCBs there then!

 

[malcolmsanford]

To be honest TD it will very much depend on the protection device on the main incomer which could be a BS 88 fuse for commercial.

If there are distribution boards around then those PD will dpend very much on the Zdb at each board

 

[MarkieSparkie]

I assume that you are measuring Ze and PFC with a MFT, at these low levels of Ze MFTs are inaccurate and a more complex test method or test equipment is required. You might like to retest using IQ high accuracy method (see the link below for details).

http://www.electriciansforums.net/i...38-higher-accuracy-low-ze-test-procedure.html

 

[GT1]

I think as others have said, perhaps where you tested isn't the origin of the return path ?
Maybe some parallel paths reducing your figure?
Are you physically able to see the earth strap coming off the casing of the incomer and confirm this what you tested ?

 

[ringer]

These measurements were made and recorded by someone else. I was just shown the PIR and had a quick look around. There are other issues with the PIR where recorded readings (or no readings were recorded!) do not ring true. Premises were in use at the time so was not possible to take my own readings. Hoping to get back soon when they are not open to do my own testing. I had remembered IQ's method for low values and intend to employ it. However I suspect that original reading for Ze was taken with bonding in place.

What I was asking originally was whether you might expect to get such a low Ze 250m away from the substation on a TN-S supply? The area is heavily built up with both commercial and residential premises.

 

[ian.settle1]

0.01/19.9 kA wow, problems with the MCBs there then!

Site I work on has it's own HV network that is why they dont use MCBs specify that it has to be fuses.

 

[GT1]

If the standard of the paperwork is generally poor then I would wait to verify that Ze reading yourself before going too deep into the ramifications of it being so low. Could easily be a clerical error with a decimal point in the wrong place. My house is on TNS and has a Ze of 0.1.
Personally not had a reading as low as 0.01.

 

[Engineer54]

Site I work on has it's own TV network that is why they don't use MCBs specify that it has to be fuses.

Why's that then?? I know why to be honest, but that spec is based on cost effectiveness rather than anything else!! Having said that, HRC fuses can get mighty expensive, if you blow a few in the course of fault finding. Especially where some, will just replace the fuse without checking for a fault first etc... lol!!!

We use 22KA MCBs in a number of cases on our projects, in fact i can't remember the last time we have used fuses on the LV systems, can't say we use many fuses on the MV side of things either, apart from in the odd RMU and in the moving MV contact arms to voltage transformers in the main MV switchboards....

 

[ian.settle1]

Why's that then?? I know why to be honest, .

You know the answer and so do I but how many people do, answers please?

 

[SPARTYKUS]

Most MCB's have a "6000" on the side...?

 

[needasparks]

You know the answer and so do I but how many people do, answers please?

Are you refering to the 80Ka on bs88's or did i miss something ?

 

[SPARTYKUS]

Breaking capacity...?

 

[needasparks]

Breaking capacity...?

sorry, was miss marples day off today.

 

[ian.settle1]

Are you refering to the 80Ka on bs88's or did i miss something ?

Clue

MCBs 6Ka rating PFC reading 19.1Ka

 

[plugsandsparks]

If it was o.o1 i would expect to see the sub in the car park to be honest or even closer, a look at the size of the incoming cable might help a bit

 

[ringer]

Customer is not rushing to sort this out, but wants to speak with me next week about it. Assuming for the moment that the measured value of Ze=0.01 ohms is correct, I am trying to establish what can potentially be done about this. To recap: TN-S, the DB circuit breakers are 6kA SCC, the DNO fuse is BS1361 type II 100A 33kA SCC, PFC is 19.9kA

So the DNO fuse will withstand the PFC, but the MCBs will not. Can we rely on the DNO's fuse to protect the installation? What about the potential for harm to the MCBs (and persons, livestock and property) at that level of fault current?

 

[ian.settle1]

Customer is not rushing to sort this out, but wants to speak with me next week about it. Assuming for the moment that the measured value of Ze=0.01 ohms is correct, I am trying to establish what can potentially be done about this. To recap: TN-S, the DB circuit breakers are 6kA SCC, the DNO fuse is BS1361 type II 100A 33kA SCC, PFC is 19.9kA

So the DNO fuse will withstand the PFC, but the MCBs will not. Can we rely on the DNO's fuse to protect the installation? What about the potential for harm to the MCBs (and persons, livestock and property) at that level of fault current?

I would read the warning that all MCB manufacturers give regarding the PFC and MCBs

 

[ringer]

I would read the warning that all MCB manufacturers give regarding the PFC and MCBs

Where would I find that warning then? These are MEM 3871 M6 type 2 MCBs. Have looked on the Eaton website, and found their MCB literature but unable to find any specific warning about excessive PFC.