TNS Ze 0.8Ohms

[GBDamo]

Did a lot of miles today and was pondering.

This is not a real scenario.

Not necessarily "how often does a Ze make an installation impossible?" but more..

If, the measured Ze is say 0.30 Ohms and you then design your installation to keep your Zs's within BBB max allowances, does the DNO have a responsibility to maintain the measured Ze or can they simply state 0.8 Ohms.

For an example you install a 10KW shower.

At time of installation the Ze is 0.3 Ohms.

Your R1+R2 is 0.1 Ohms, Zs of Approx 0.4 Ohms.

10mm T&E on a B50 Zs limit = 0.69 Ohms.

So, job done, cert issued and notified.

Stay with me....

6 months later there's a change of tenancy and the landlord has an EICR done. I nkow

Now the Ze is 0.6 Ohms and my installed circuit is measured as 0.7 Ohms and is listed as a C2 on the cert.

What happens, will the DNO just state "0.8 Ohms not our problem"

 

[Julie.]

If the Ze is within the prescribed limits, then there is no responsibility wrt the dno. Just like if they provided 240V on your day of testing, but 225V thereafter - all are within the range allowed under their obligations.

The installation we design should comply with the ranges allowed, so for example if I design for a site that has a pme of 0.2ohm, I would still design for 0.35ohm, as this is the limit (or 0.8 ohms for your TNS example).

This means that on some designs I have to specify rcd/rcbo for fault protection (rather than the sometimes optional additional protection), even though on the day, theoretically the actual Zs is below that needed to operate a MCB within the timescale needed, because just like we have to design for the limits of voltage allowed, we ought to design for the Ze allowed.

If the dno then changes the earthing pme->TNS or just out of the prescribed range, it would be their responsibility to fix, or advise the client of the changes.

As for the degrees of accuracy of our measurement, a different point of course and contentious as if I measure 0.34ohm but my megger has an accuracy of +/- 0.05ohm, then actually I can only be aware that the Ze is between 0.29 and 0.39 ohm, which could be in spec' or not!

 

[davesparks]

The installation we design should comply with the ranges allowed, so for example if I design for a site that has a pme of 0.2ohm, I would still design for 0.35ohm, as this is the limit (or 0.8 ohms for your TNS example).

0.35 is not the limit for PME, it is a typical maximum value for most supplies.

If you look at UKPNs specifications for example they state a typical maximum of 0.35ohm for an older 100A supply but that they design all new 100A supplies to a limit of 0.25ohm.
Obviously these figures then go down as the supply size goes up.

 

[Pretty Mouth]

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.

 

[Julie.]

0.35 is not the limit for PME, it is a typical maximum value for most supplies.

If you look at UKPNs specifications for example they state a typical maximum of 0.35ohm for an older 100A supply but that they design all new 100A supplies to a limit of 0.25ohm.
Obviously these figures then go down as the supply size goes up.

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.

If I used 0.2 ohm, and it failed to clear in time, and I have to justify my design I could be up a certain creak sans paddle. Using the published limits from the infamous book, I have a valid defence.
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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.

Not necessarily, the main fuse would limit the actual let-through at those sorts of current levels, but it becomes a more complicated calculation than we would normally use.

 

[Vortigern]

The correct voltage is >50v being the highest amount of touch voltage that is acceptable. Therefore the figure of 1667 ohms max.
i.e. RA x Ideltan less than or equal to 50v. 411.5.3

 

[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.