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Twiggy00_00

Maximum Zs

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Twiggy00_00

Hi all

Is there different amounts and table for a Tns system to a tncs reguarding the mcbs maximum Zs. 

I have a Ze reading of 0.40 which is fine but all my circuits have a high Zs reading. 

Eg. Two cooker circuits both 40A type B mcbs are getting a reading of 1.22

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ProDave

What's R1+R2 on these circuits with a high Zs?  Something sounds wrong to me. Very long runs or faulty isolator switch perhaps.

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Twiggy00_00

R1+R2 for these were 0.12

all circuits are fine otherwise 

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Andy™

tried changing batteries in tester?

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Twiggy00_00

No. Think they’re ok as they’re fairly new

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kerching

Leads zeroed correctly?

batteries ok?

good firm connection made to,wires?

meter warmed up and stabilised?

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Sharpend

Where are you testing? 

On wires directly or via a plug socket? 

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Rob_the_rich

Is there an Earthing cable between the earth bar in the CU to a separate MET where perhaps you tested Ze?

Could be a dodgy cable or connection

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Murdoch

I would remove the cooker isolator and connect your test leads direct to the cable ...... then re test.

 

Unless you have  Metrel and its leads are knackered....

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Hefferl

Is an RCD in the circuits causing these high Zs?

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Twiggy00_00

Testing using the lead and the plug on every circuit. 

I take it my mcbs have the same max Zs regardless of tncs or tns? 

 

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binky

 I would work on 1cct like the cooker, get R1+R2 to double check cct, take Zs direct from cables, not through the accessory plate, if  Zs is still high repeat at MCB top and bottom of breaker (ie a Ze type test), check all terminals on MCB, main switch and any RCDs are tight. Check busbar is installed correctly. 

 

If using a plug-in test lead it is not unusual to get higher readings, plug in and out several times to remove any corrosion, switch on and off several times to clean contacts, then re-test.

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Twiggy00_00

Thanks Binky. I’ll give it all another test and prove the meter as well first

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Doc Hudson
1 hour ago, Twiggy00_00 said:

Testing using the lead and the plug on every circuit. 

I take it my mcbs have the same max Zs regardless of tncs or tns? 

 

 

Do you understand the concept of what the maximum earth fault loop impedance is for, or have you read the section 411.4.xx in BS7671. If you are using MCB's to achieve the disconnection times, to comply with 411.3.2.2. in the event of a fault of negligible impedance between Line and earth, then you must not exceed the maximum Zs values. If you go back and re-read the appropriate sections you will answer your own question.  It sounds as though you are getting confused with TT systems where additional protection is required because of the high external earth loop impedances due to the lack of a direct physical earth path back to the supply transformer. 

 

Doc H.

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