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"

 

[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!