What’s your countries Max Zs Values?

[KennyKen]

I’m intrigued to see the differences in Max Zs values compared with the 18th & NEC

 

[pc1966]

What is interesting there is they are higher then BS 18th and the method generally used here.

For example, a 32A B-curve MCB here has the "instantaneous" magnetic trip at 3-5 * In so 96-160A and we would assume the worst-case, so to hit the sub-0.4s time you need Zs to be below 230V / 160A = 1.44 ohms. This is already below your 1.8 by a noticeable amount!

Next we assume you have to meet this at lower limit of UK supply voltage specification (95% of 230V) and when the cables are hot (so if measured cold, 80% of worst-case resistance) leading to a verification value on cold testing of:

1.44 * 0.95 * 0.8 = 1.09 ohm (given in the IET's On-Site Guide Table B6 as 1.1 ohm)

So Aus/Nz are 64% more optimistic!

 

[telectrix]

australians are more sensible. when do we run cables @ 70deg.c? and when is our voltage down to 216V? load of crap. just anothe reason for IET to bring out a new edition to increase their pension funds at our expense.

 

[Welchyboy1]

Interesting theres quite a disparity there and we are supposed to only allow 80% of the stated values too!

 

[Vortigern]

Well if we use an RCD it is acceptable to allow a Zs of 1667 ohms so pretty high really.

 

[pc1966]

Interesting theres quite a disparity there and we are supposed to only allow 80% of the stated values too!

The Table 41.3 in the BBB is for the MCB at Umin and no allowance for cable temperature, so in that case we have for 32A B example:

Zs = Umin / Itrip = (0.95 * 230) / (5 * 32) = 1.3656 (tabulated as 1.37 ohm)

But if you look in Table B6 of the On-Site Guide thay have allowed for a factor of 0.8 already (i.e. typical assumption for cables operating at 70C)

Zscold = Zs * 0.8 = 1.3656 * 0.8 = 1.09248 (tabulated as 1.1 ohm)

Where Zscold is what you measure dead, assuming not immediately after shutting the load off!

Basically the OSG is really looking at the most common case for PVC cable near their rating, where as the BBB table is expected (I assume) to be more generic so if you know your cable is very cold or hot from other design aspects you can apply an appropriate factor to it when designing the circuit.

[automerge]1596965103[/automerge]

Well if we use an RCD it is acceptable to allow a Zs of 1667 ohms so pretty high really.

I have never been happy with the use of an RCD as your primary protection.

I.e. instead of "additional shock protection" when the MCB (or equivalent in RCBO) is able to clear a fault on Zs.

Yes, for years that has been the case with a TT supply but ideally we would have TT systems based on a 100mA/300mA delay incomer and then 30mA RCD for additional personnel shock protection so there is no single point of failure in ADS protection.

So while the regs allow for this, I would only really see it as good practice in cases where there are no sane alternatives.

 

[Vortigern]

I do agree @pc1966 as it does not also cover a L-N fault. For that reason it is not a short cut I would be prepared to take although I have seen it done and certs filled in with Max Zs 1667 ohms, not a good practice.

 

[LastManOnline]

Well if we use an RCD it is acceptable to allow a Zs of 1667 ohms so pretty high really.

Agreed. Good example of what the regs at times allow but what in reality would amount to poor (in this case even bad) work practice. Add to that the fact rod resistance can vary considerably.

 

[pc1966]

Agreed. Good example of what the regs at times allow but what in reality would amount to poor (in this case even bad) work practice. Add to that the fact rod resistance can vary considerably.

While the UK regs allow for 1667 ohm in theory (i.e. 50V touch potential at 30mA) they also state that earth rod impedances above 200 ohms are likely to be unstable in practice.

So in reality if you see Zs above 200 ohm then it is bad even for 30mA RCD!