Maximum ZS values GN3

[newfutile]

On page 142 of the latest Guidance Note 3 it list a 16 amp type B max ZS as 2.15, however i think 2.2 is the real value, it also states that at 16 amp type C is 1.1 ohms which is correct, but a type B should be double the value of a type C.

anyone else have a view on this?

 

[davesparks]

What makes you think a B type should be double the value of a C type?

 

[newfutile]

BS 60898 Type B devices are designed to trip at fault currents of 3-5 times rated current (In). For example a 10A device will trip at 30-50A. Type C devices are designed to trip at 5-10 times In.
we take the worst case of 5 times for type B and 10 times for type C this is how the maximum ZS is calculated . therefore as Z is inversely proportional (Ohms Law) Double the Amps will have to half the impedance Z for the same voltage.

 

[Julie.]

Yeah, I think it is wrong, the required current for 16A B curve would be 80A

So 230 x 95% (voltage) x 0.8 (cold conductor) /80A is 2.185 ohm

I think it's been rounded off a bit too much!

Btw a B type will only be double the C type resistance for generic characteristics, once you obtain manufacturers data it doesn't always maintain this "rule" because the instantaneous section of the curve isn't necessarily at 5 and 10x - there is actually a broad acceptable range so you could have a B with 4x, and a C at 11x

 

[newfutile]

yes i would agree with that, you could also look at a 32 Amps type B at 1.1 ohms , based on your calculation it looks like that missed the 8 out

 

[Julie.]

yes i would agree with that, you could also look at a 32 Amps type B at 1.1 ohms , based on your calculation it looks like that missed the 8 out

Looks right to me
230V x 0.95 x 0.8 /160A = 1.093 ohm (~1.1)

 

[newfutile]

yes i agree that this one is correct , the point i was trying to make is the 16 should be double the value of the 32

 

[Cookie]

Yeah, I think it is wrong, the required current for 16A B curve would be 80A

So 230 x 95% (voltage) x 0.8 (cold conductor) /80A is 2.185 ohm

I think it's been rounded off a bit too much!

Btw a B type will only be double the C type resistance for generic characteristics, once you obtain manufacturers data it doesn't always maintain this "rule" because the instantaneous section of the curve isn't necessarily at 5 and 10x - there is actually a broad acceptable range so you could have a B with 4x, and a C at 11x

Wouldn't you assume a conductor AC impedance based on 70*C as that is 1) the worst case 2) reactance is ignored 3) wire gets hotter as the short circuit occurs.

 

[Julie.]

Wouldn't you assume a conductor AC impedance based on 70*C as that is 1) the worst case 2) reactance is ignored 3) wire gets hotter as the short circuit occurs.

No

You are measuring the Zs when cold so you have to correct for running temperature.

You need to ensure, in this case it operates at 80A, well at 230V this would be an impedance of 230/80 ohm.

However, the voltage is allowed to be lower than 230V by 5% so to ensure it still operates, you must use 95% of nominal voltage hence 230 x 95% / 80 ohm.

This would be the impedance when hot, but we will be measuring it whilst cold, therefore following the normal accepted figures you take 80% to be the cold value.

This obviously corresponds to a temperature rise from cold (20 degC) to 93 degC

Hence the formula I posted (which is laid out in the standards anyhow)

 

[Cookie]

No

You are measuring the Zs when cold so you have to correct for running temperature.

You need to ensure, in this case it operates at 80A, well at 230V this would be an impedance of 230/80 ohm.

However, the voltage is allowed to be lower than 230V by 5% so to ensure it still operates, you must use 95% of nominal voltage hence 230 x 95% / 80 ohm.

This would be the impedance when hot, but we will be measuring it whilst cold, therefore following the normal accepted figures you take 80% to be the cold value.

This obviously corresponds to a temperature rise from cold (20 degC) to 93 degC

Hence the formula I posted (which is laid out in the standards anyhow)

Thanks, that clears it up. I'm used to table 9 chapter 9 which assumes 75*C conductor temps.

One last question... why not -10% voltage?

 

[telectrix]

in UK the permitted variation of supply voltage is 230V + 10% to 230V -6%. i.e. 253V down to 216V

 

[Julie.]

Thanks, that clears it up. I'm used to table 9 chapter 9 which assumes 75*C conductor temps.

One last question... why not -10% voltage?

the tables in the standard are used for design - and use running temperatures, however the tables in GN3 are for test and verification, so are done cold. Hence differences between GN3 the OSG and the BS standard.

It should really be -6% but the standard uses a rounded off value of 95%

There is another issue with SWA-XLPE/tri-rated cables, in that they run at higher temperatures, whilst the standard figures/corrections are based on 70 degC.

Therefore I always size/calculate suitable protection/cable size based on 95% of the usual figures as the higher running temperature results in a higher resistance.

So if the MCB on XLPE or tri cable needs a Zs of 1.1ohm - I would always ensure it has no more than 95% of this - plus a safety factor as usual.

 

[davesparks]

There is another issue with SWA-XLPE/tri-rated cables, in that they run at higher temperatures, whilst the standard figures/corrections are based on 70 degC.

Therefore I always size/calculate suitable protection/cable size based on 95% of the usual figures as the higher running temperature results in a higher resistance.

So if the MCB on XLPE or tri cable needs a Zs of 1.1ohm - I would always ensure it has no more than 95% of this - plus a safety factor as usual.

But if its an MCB then it will almost certainly only have terminals rated for 70degree operation, so the cable will be designed based on its operation at 70.

 

[Cookie]

But if its an MCB then it will almost certainly only have terminals rated for 70degree operation, so the cable will be designed based on its operation at 70.

Touche! Do they make 90*C terminal breakers in the UK?

 

[Julie.]

But if its an MCB then it will almost certainly only have terminals rated for 70degree operation, so the cable will be designed based on its operation at 70.

Correct, but if we end up using the additional capacity, then i apply the fudge factor.

Although I suspect there are many out there that just use the rating from the table as-is without either considering those ratings apply only if you can use the higher temperature rise, nor the higher resistance when operating.

The additional resistance thing is just another factor that is easily forgotten, it doesn't replace normal design practice, just part of it.
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Touche! Do they make 90*C terminal breakers in the UK?

Yes, but usually mccb, fuse switches, or just connectors.

If you are just feeding it off an mcb in a standard board, then not as far as I know.


The other thing though is the temperature rise may not be for the full length.

If the cable runs through hot ambient areas, or through good thermal insulation then the whole length has to be downrated, - if this was 50% then yes a 100A cable may be running at 90 degC in the key area, but the actual terminations wouldn't be anything like that as it's only 50A through a 100A cable so you can use the full temperature capacity of xlpe, even with normal temperature range terminations.

It just takes a bit more thinking about, and calculation

 

[Vortigern]

Then add to that the figures obtained in Zs test are maybe +- 6 digits, and the fractional ohms difference is far more skewed potentially by that digit difference then you are arguing about how many angels fit on a pin head. In the real world when there is a fault of negligible impedance the fault current would likely be far above the figures talked about here. So usually a more pragmatic view is taken on the ground, so to speak, and the figures are accepted on that basis give or take a gnats b_____ks. In short panning back from the theory, at this level at least, the figures are close enough. Again in practice, the calculated Zs, say TN-C-S 0.35 assumed, + resistance tables =.55 (say) assumes 0.90, when measure you get 0.66, and a six digit difference means it could be 1.2 ohm down to errm well you work it out.