TNS Ze 0.8Ohms

[R-fur]

Back in the 80s I was an installation inspector for the local electricity board and before fitting a meter we had to check that the earth loop impedance was below 0.8/0,35 ohms.
You should always design to the maximum stated value as the DNO may reconfigure their network without notifying the customers.
A typical example is the PSCC the 16kA is based on you being right next to a substation with a 3 meter long service cable. You may know that the substation is a 300 meters away the DNO will say that they may but a new substation right next the house in 10 years time.
Some DNOs allow you to reduce the 16kA if you know the distance between the point of supply and the nearest footpath, again even if you know the main cable is across the road, it may be moved in the future. Appendix 14 says you do not need to measure PSCC in a domestic installation if the DNO declare it at 16kA.
My understanding is that you can use 6kA mcbs as long as they are in a type tested enclosure backed up by the DNO fuse, cant remember where that is written down though.

 

[Julie.]

Back in the 80s I was an installation inspector for the local electricity board and before fitting a meter we had to check that the earth loop impedance was below 0.8/0,35 ohms.
You should always design to the maximum stated value as the DNO may reconfigure their network without notifying the customers.
A typical example is the PSCC the 16kA is based on you being right next to a substation with a 3 meter long service cable. You may know that the substation is a 300 meters away the DNO will say that they may but a new substation right next the house in 10 years time.
Some DNOs allow you to reduce the 16kA if you know the distance between the point of supply and the nearest footpath, again even if you know the main cable is across the road, it may be moved in the future. Appendix 14 says you do not need to measure PSCC in a domestic installation if the DNO declare it at 16kA.
My understanding is that you can use 6kA mcbs as long as they are in a type tested enclosure backed up by the DNO fuse, cant remember where that is written down though.

434.5.1


Plus some words it would appear

 

[R-fur]

Cheers mate, I should have known that, spending today swotting as I am doing the 18th edition updates tomorrow and Thursday.

 

[Julie.]

Even a typical let through for BS88 fuse

You doing the 18th or 18th amendment 1?

 

[R-fur]

Doing SELECT 209 initial verification and 214 periodic inspect and test. Not too bothered about amendment 1 as I have never got involved with vehicles, yet.

 

[Risteard]

Agree, but if the limit as defined in the 18th is used, then just like everything else, I have a valid defence in court should something happen.

But BS7671 does not define any limit. In fact it is wholly outside the scope of BS7671.
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Interesting topic. If we were to design according to our DNO's quoted values, rather than measured, this would presumably include the max quoted fault current. In Western Power's case this is 16kA (single phase, LV), which would mean I would be unable to fit standard 6kA breakers in all circumstances.

The conditional rating arising out of Annex Zb (previously Annex Za) means that is not the case.

 

[davesparks]

Agree, but if the limit as defined in the 18th is used, then just like everything else, I have a valid defence in court should something happen.

Where is it defined in BS7671?
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Interesting topic. If we were to design according to our DNO's quoted values, rather than measured, this would presumably include the max quoted fault current. In Western Power's case this is 16kA (single phase, LV), which would mean I would be unable to fit standard 6kA breakers in all circumstances.

Incorrect, the cutout fuse provides backup protection for the potential 16kA as long as those 6kA devices are in a suitable type-tested assembly

 

[Julie.]

But BS7671 does not define any limit. In fact it is wholly outside the scope of BS7671.
……..

Oops, forgot it's actually only in the on-site guide (and perhaps GN3), I know the quoted figures were actually defined in ER P23/1 - with caveats for rural sites of course, and that these are no longer the values since the introduction of P23/2 recently, but my point is that if I adhere to proper guidelines I am better covered than hoping for a low value on the day.

 

[Pretty Mouth]

I stand corrected! Can someone please point me to where I could read a bit more about this. Can't seem to find anything about it in the BBB, OSG or GN3.

 

[R-fur]

434.5.1 as Julie said. Section 3.3 OSG, section 3.5.3 of GN1? I have an old copy so this may not be correct.

 

[Pretty Mouth]

434.5.1 as Julie said. Section 3.3 OSG, section 3.5.3 of GN1? I have an old copy so this may not be correct.

Thanks . . .

 

[Midwest]

I stand corrected! Can someone please point me to where I could read a bit more about this. Can't seem to find anything about it in the BBB, OSG or GN3.

Believe you might find it the specification for BS EN 60947 or is it BS EN 60898, and reg 434.3 (iv) if your on about 16kA with 6kA mcb's, me thinks.

 

[radiohead]

I also didn't think you could rely on an RCD for fault protection if it is TNS or TNCS. Zs on these systems would almost become irrelevant if we did as they would always be below 1667.

Clearly that maximum Zs value would not be applied to a TN fed circuit despite being compliant. If the measured Zs got much beyond the value that would be expected then other factors in the circuit design or earthing system would be suspect. For example lets say a measured Zs of 100 ohms is recorded on a standard ring circuit, either the Ze is way beyond acceptable limits, or the circuit either has a fault or is about a mile long.
Where the 1667 value becomes useful is where it is desirable to change a type B to a C or D, because inrush currents are causing nuisance tripping for example. Often an RCD will allow that when the OCPD max value will be exceeded.

 

[Julie.]

I stand corrected! Can someone please point me to where I could read a bit more about this. Can't seem to find anything about it in the BBB, OSG or GN3.

About which bit?

The 0.35 ohm for PME and PNB is in the osg tables 7 or 10 - not handy at the moment so just memory.

The new figures are <0.34ohm for 90% of properties <0.64 for 98% of properties and >0.64 ohm for up to 2%, but the IET advise to continue with the existing practice - 0.35 ohm, with confirmation by measurement that it is less or equal to this. (can't remember the actual complex values but I think it's 0.25+j0.23 ohm for 90%)

As for the coordination of upstream fuses and their downstream let through to allow for lower breaking capacity devices downstream; the regulation is in 434.5.1.

Whilst the methodology is just standard coordination - as per type c coordination and others.

Download any fuse manufacturers data and they should include the let through current vs prospective current chart - I included one example previously.

Essentially for circ 100A fuses 6kA downstream kit would be suitable for up to circ 20kA prospective fault level, but this drops of for bigger fuses, so 16kA would hit the 6kA limit with circ 250A fuses.

 

[Pretty Mouth]

Thanks all. I downloaded the same data you posted Julie and I get it now.

There's quite a lot to this electricity malarky isn't there.

 

[Julie.]

Thanks all. I downloaded the same data you posted Julie and I get it now.

There's quite a lot to this electricity malarky isn't there.

Yup, the more you find out, the more you find out you don't know!

 

[HappyHippyDad]

It would be nice if they produced a book that gives clear guidance and limits, maybe review it and ammend it every year or two, it would be costly mind but it would remove the ambiguity.

Please.... no more books!