earth loop impedance testing

[sparky43]

Do i have to do a Zs test on sockets that are fed from a centre tapped 110V transformer. I can't find any reference to the max Zs values of a nominal voltage of 55V to earth.

any help appreciated.

sparky

 

[pugwash]

Sparky,

Short answer is - Yes you do.

Assuming you're UK based, the relevant section in the Wiring Regulations (17th Edition) is 411.8 and table 41.6. It's basically the same calc as for a 230V system, thus Max Zs for:
type B breaker at 55V = 11/In Ohm
type C breaker at 55V = 5.5/In Ohm
type D breaker at 55V = 2.8/In Ohm
where In is breaker rating

HTH

 

[sparky43]

Hi Pugwash

i managed to find the info in the 16th edition regs table 471A gives me the info i needed & this link helped.

http://www.tlc-direct.co.uk/Book/7.16.4.htm

thanks for the help and the information.

regards
sparky

 

[pugwash]

Hi Sparky,

Principles are the same, just watch the values may be slightly different between type 2 and type B for instance.

Also don't forget that the maximum ambient test figures should be 0.8x these figures to take into account the increase of resistance of the conductors with the increase of temperature due to load current (17th - Appx14). E.g. The max Zs for a 10A type C breaker should be 0.8 x 0.55 = 0.44 Ohms, when measured 'cold'.