Loop Impedance

**C-Cam** · 25-Apr-12, 05:50 PM

So glad to find a place where I can ask questions.

Is there a chart or table that can help me determine the acceptable loop impedance and PSU results on an existing installation.

**Mojakoki** · 25-Apr-12, 10:58 PM

From SANS 10142-7 : 8.7.5, At the main switch, the impedance shall be such that an earth fault current double the rated current (or higher) of the main protective device automatically disconnects the supply to the installation. The formula
for a 60 amp supply : voltage over 2 x current ( V/2I ) ie 230 divided by 120 = 1.92ohms. A reading less than 1.92ohms is acceptable. (p.s. post again if you need this test described in laymans terms)
Your second question - I presume you mean PSC (prospective short circuit)(rating). What is the rating of your breakers?
Probably 5kA or 5000amps (marked on the breaker) - your reading must be less than that.

**AndyD** · 26-Apr-12, 04:29 AM

Welcome to the forum Mojakoki

**C-Cam** · 26-Apr-12, 05:17 AM

Thanks! I was recently told by the council we can't accept readings above 1.4ohms. Got a bit confused.

On 3 phase - would I then use 380V or 230V.

**Mojakoki** · 26-May-12, 05:56 PM

If your reading is high, (too high for your Council's liking), consider installing an earth rod.
Pretty sure you would use 230V for a 3 phase installation as you are testing the earth resistance back to the transformer where the suppliers earth is supplied.

**CGelectrical** · 28-Dec-20, 10:16 AM

Hi Guy's,

Anybody that can assist with a excel sheet for the calculation of Loop impedance and Ipscc. I have a warehouse where between council and client i have 240mm x 4c cable about 10m. From there i have 3 x 150mm 4c cables to main mcc about 70m. Need to calculate. I have all the transformer information.

Thanks in advance.

Conrad

**RegElec** · 20-Jan-21, 03:00 PM

Originally Posted by CGelectrical

Hi Guy's,

Anybody that can assist with a excel sheet for the calculation of Loop impedance and Ipscc. I have a warehouse where between council and client i have 240mm x 4c cable about 10m. From there i have 3 x 150mm 4c cables to main mcc about 70m. Need to calculate. I have all the transformer information.

Thanks in advance.

Conrad

Hi

I have a Schneider slide rule on spreadsheet with explanations on how to read it as well as other interesting tabs for transformer currents etc. Its over 2MB so I cant attach it here. If you can send me mail I will send it to you. I have a second spreadsheet that I have compiled that I can send. The formulas have not been confirmed by an engineer but so far the answers have been correct.