Max Z's for 32A Type C MCB
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0.58
Its 0.72 ohms which you should enter on your certificate or report but you should compare the measured value against the 80% corrected figure as posted by Davy.
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Cheers,
The actual reason I needed the figure was because this morning I had to install a digital signage TV.
The supply that I was instructed to connect to was what I can only describe as a dodgy looking spur. Upon testing with my loop meter I found Ze to be 0.88 and as stated this is on a type C 32A breaker.
"Luckily" the contractor sparks were on site and I asked them to test it, within five mins I got a call on my radio to say it was OK but I know for a fact that they never even looked at the dist board and I never told them that it was on a type C breaker...
What else is on the circuit? Is it a spur off a ring final? Is there any reason why the MCB cant be changed to a B32?
Hi,
Yes its a spur from a ring final, the ring is a supply for mainly unused sockets in a seating area.
It could be changed for a B32 as I dont think there are any significant loads.
I will make the recommendation to the organ grinders.
Its very easy to calculate Zs yourself. You dont need a reference book.
Type C breaker requires X10 rated current to meet 0.4 & 5 second disconnect time, voltage is 230.
You have current and voltage so just use Ohms law to calculate impedance:
230V / 320A = 0.718 Ohms rounded to 0.72 Ohms
80% of 0.72 = 0.576 rounded to 0.58 Ohms
regards
Seven replies 
Good I should think.
I should point out that i'm not an electrician, my work primarily involves data and audio visual cabling. I only ever use the tester to try and make sure anything that i am installing is safe.
I have just started my first year of C&G 2330 and feel that this site and threads like this have helped to prepare me for it.
Having said all of that, why the 80% correction factor?
Matt
It is to take into account the difference of the resistance of the cable between its maximum operating temperature (70deg) and the temperature when it is measured (10deg).
The 60 degrees difference is fed into the equation 1+(0.004x60) = 1.24. (0.004 being the simplified temperature coefficient of copper), divide the tabulated value by 1.24, or multiply it by its reciprocal which is nigh on 0.8.
The 60 degrees difference is fed into the equation 1+(0.004x60) = 1.24. (0.004 being the simplified temperature coefficient of copper), divide the tabulated value by 1.24, or multiply it by its reciprocal which is nigh on 0.8.
It just seemed a random comment - Seven replies
Measured values of earth fault loop impedance are compared to 80% values when the initial measurement is made at ambient temperature.
Under load conditions the conductor starts to get warm, with heat, resistance increases. As resistance increases the measured earth fault loop impedance figure you took will rise.
By designing a circuit to an 80% figure you leave a little headroom for rises in temperature and resistance due to loading.
Hope this helps a little.
EDIT - Beaten once again by Spark and mathematics
Measured values of earth fault loop impedance are compared to 80% values when the initial measurement is made at ambient temperature.
Under load conditions the conductor starts to get warm, with heat, resistance increases. As resistance increases the measured earth fault loop impedance figure you took will rise.
By designing a circuit to an 80% figure you leave a little headroom for rises in temperature and resistance due to loading.
Hope this helps a little.
EDIT - Beaten once again by Spark and mathematics
I'm guessing he means Zs as he took his measurement at the spur.
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