Zs reading

[butler26]

Hi

Did a Zs reading today on a cooker circuit protected by a rcbo the reading I got using Megger 1552 was 0.18 ohm but when u add R1+R2 + Ze I got 0.14 ohm Ze is 0.08 and R1+R2 is 0.06 . Shouldn't the measured value be lower than the calculated value

Thanks

 

[batty]

Why should it be lower remember low current loop testers especially Megger are not that accurate.

 

[kme]

0.06 on a cooker circuit, for R1+R2 ????

That is some short run, and/or a huge bit of cooker cable!!!!!

 

[Revved Up Sparky]

I sometimes get R1 + R2 readings that low for a 6.0mm fed cooker circuit that is sited close to the consumer unit, on average though it is about 0.11 - 0.15 ohms.

Nothing wrong with that Zs in my opinion. It will not always be lower than Ze + (R1+R2) , for example if there are no parallel paths it may be pretty much the same.

 

[kme]

It is also well known & documented that OCPDs; particularly RCBos, can introduce a quantifiable impedance - they are the one thing that is generally additional to the chain.

i.e. the OCPDs inherent resistance isn`t measured with Ze; nor is it (usually) in the R1+R2 figure.

KME

 

[butler26]

Thanks for the reply lads ,the cable is 10mm and about 10 meter run well I feel better now knowing that it is alright

 

[PC Electrics]

In reality the equation would be:

Zs = Ze + (R1+R2) + 'losses'

Losses could be resistances in the connections, main switch, busbar, RCBO etc. You do not measure these quantities, you measure R1+R2 on the circuit only and Ze external to the CU, so any internal resistances to the CU will add to the measurement. IMO a difference of 0.04 is insignificant in most cases - you might check it more carefully if the circuit was right on the edge of max Zs for the ocpd.