Table 4.1.2a/b in Electricians Guide

[grahambulmer]

Could somebody please clarify the difference between tables 4.1.2a & b. It appears to me that the difference is in types of circuit breakers - B or C depending upon max test loop impedance. Is this correct and which measurement do they mean by test loop impedance?

 

[Wilko]

Yes, type B and C type breakers respectively. Here's a nice intro to loop impedance

 

[telectrix]

basically, a type C requires double the Amps of a type B to trip within the specified times, so the max. Zs is halved.

 

[westward10]

A circuit breaker will disconnect within 0.1 sec whenever the fault current is equal or greater than the upper limit of their instantaneous tripping band.
For Type B devices this is 5 × In.
For Type C devices this is 10 × In.
The magnitude of the fault current is dictated by the impedance of the fault path which can be measured by an earth fault loop impedance tester. The lower the impedance the higher the fault current and as Type C devices require higher current for any given time the maximum required Zs is lower.

 

[Octopus]

Could somebody please clarify the difference between tables 4.1.2a & b. It appears to me that the difference is in types of circuit breakers - B or C depending upon max test loop impedance. Is this correct and which measurement do they mean by test loop impedance?

Why are you asking the question? College work?

 

[grahambulmer]

Thanks everyone, I understand it now.