Zs compliance. Example questions.

[richardindorset]

Todays class ended in a bit of a fiasco trying to work out how to check compliance for measured Zs, when given various protective devices.

For example: A circuit is protected by a 20A BS88-2 fuse and measured Zs is 2.08 ohms. Ambient temp is 20 degrees C.

I approached this question using the OSG. Looked up the max Zs in table B3, 0.4s disconnection time, 1.3 ohms

Then apply temp factor in table B8 of 1.04 so the max Zs becomes 1.352 ohms maximum.

Therefore in this case 2.08>1.352 and the circuit is not compliant.



If we use the Regs book, we go to table 41.2 and find a max Zs of 1.68 ohms. This is higher due to being set at 70 degrees C.

The regs book also says you can use a formula in Appendix 3 where measured Zs must be less than or equal to 0.8 x ((Uo x Cmin)/Ia), which in this case would be ((230*0.95)/20)*0.8 = 8.74 ohms.


Which is the correct approach? Obviously the lower the value, the better, but I'd be interested to know which is correct. Our lecturer couldn't explain it.

Cheers!

 

[EFLImpudence]

The regs book also says you can use a formula in Appendix 3 where measured Zs must be less than or equal to 0.8 x ((Uo x Cmin)/Ia), which in this case would be ((230*0.95)/20)*0.8 = 8.74 ohms.

Ia is the current causing operation of the protective device within the specified time.

That would be 100A for a 20A B MCB; i.e. 5 times 20.
Not sure what it is for a 20A fuse. Is it 7 times 20?

 

[richardindorset]

Ia is the current causing operation of the protective device within the specified time.

That would be 100A for a 20A B MCB; i.e. 5 times 20.
Not sure what it is for a 20A fuse. Is it 7 times 20?

Ah! OK, that makes a lot more sense. So a BS88-2 would be 130A in 0.4 seconds.

So 1.344 ohms. Very close to the OSG figure.

Reading the Guidance Note 3 book, it suggests four methods to decide if the measured Zs is acceptable.

 

[richardindorset]

After some more reading I see Zs(m) mentioned. This is the value of Zs measured.

So when doing verifiaction of measured Zs, one should always use the values in the OSG, or the BS7671 regs book * 0.8 (80%).

This makes total sense, even if C&G don't seem to do it...

One part I am still not sure about. There is a temp compensation table in the OSG, table B8. This should also be used to offset from 10 degrees ambient temp, correct?