2392 Question

[rustynails]

A circuit has an external loop impedance Ze of 0.6. The maximum value of (R1 + R2), to BS7671:2008, on a circuit protected by a Type B 32A circuit breaker giving a 0.4 second disconnection time will be:

A: 1.44
B: 2.04
C: 0.84
D: 0.24

I got this wrong but have since worked it out I think.

To calculate Zs you need Ia

To get Ia you multiply breaker type by 5 in this case and get 160, confirmed on regs p301

then

Zs = Uo / Ia

Uo = Nominal Voltage = 230
Ia = Current causing the operation of the protective device within the specified time = 160A

So...

230 / 160 = 1.4375

Not quite so I guessed and minused the Ze of 0.6

1.4375 - 0.6 = 0.8375

I can't explain why if this is right?

 

[ackbarthestar]

Zsmax (1.44*0.8)for a given MCB (table 41.3) <= Ze (0.6) + R1+R2(0.55)
So 0.55 Ohm is the real answer

C&G answer is 1.44 <= 0.6 + 0.84

O.84 Ohms

This could be confusing since this answer makes no allowance for ambient and operating temperatures.
Also to get the answer it encourages you to use R1+R2 <= Zs -Ze which is not advisable because it may include, in reality, earth parallel paths.

 

[adiewadie247]

Zs = Ze + (R1+R2)

so

(R1 + R2) = Zs - Ze = 1.4375 - 0.6

 

[rustynails]

It still confuses the hell out of me. To calculate the Zs for the circuit in question you would need to establish the Ia of the protective device.

How do you calculate Ia without a book giving you the max Zs for the circuit protective device?

 

[JUD]

Zs = Ze + (R1+R2)
Max Zs = Uo ÷ Ia
Max Measured Zs = (Uo ÷ Ia) x 0.8 (rule of thumb to correct for conductor operating temperature)

For BS 60898/61009 circuit breakers Ia is easy..

Type B = 5 x In
Type C = 10 x In
Type D = 20 x In

For other types of OCPD you use the time/current characteristics graphs in App 3 of BS 7671

 

[rustynails]

For other types of OCPD you use the time/current characteristics graphs in App 3 of BS 7671

What about without the regs book?

 

[ackbarthestar]

I think JUD's answer is clearer than mine.

Go through it step by step until the 'penny drops'

All the info to do the calculation is there.

To calculate the max Zs, rather than using the tables you have type B = 5*In, type C= 10*In and Type D = 20*In for each type of MCB are rules of thumb given to you by BS7671.

 

[JUD]

What about without the regs book?

With regards to 2392 all the information you need will be in the questions.

Outside the 2392 you should always have the BGB or On-site Guide to hand.

 

[telectrix]

as jud says. max. Zs is 230/ ( 5 x Ia ) = 230/160 = 1.44 ( for a 32A type B )

 

[rustynails]

With regards to 2392 all the information you need will be in the questions.

That's good to know.

Outside the 2392 you should always have the BGB or On-site Guide to hand.

Sound advice.