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?