Maximum Zs

[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

 

[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.

 

[Twiggy00_00]

R1+R2 for these were 0.12

all circuits are fine otherwise 

 

[Andy™]

tried changing batteries in tester?

 

[Twiggy00_00]

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

 

[kerching]

Leads zeroed correctly?

batteries ok?

good firm connection made to,wires?

meter warmed up and stabilised?

 

[Sharpend]

Where are you testing? 

On wires directly or via a plug socket? 

 

[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

 

[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....

 

[Hefferl]

Is an RCD in the circuits causing these high Zs?

 

[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? 

 

[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.

 

[Twiggy00_00]

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

 

[Doc Hudson]

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.

 

[SirKit Breaker]

R1+R2 for these were 0.12

all circuits are fine otherwise 

Dont know if you have resolved your problem, but, if you have R1+R2 value of 0.12 and a Ze of 0.40, then your Zs cant be any higher than 0.52. maths dont lie. You say that you have 2 circuits giving you a reading of 1.22, you are highly unlikely to get 2 identical results unless you have 2 identical circuits in all respects, such as length, temperature and any other condition you can think of. The batteries dont come into play unless they are dead enough for it not to turn on, Zs is a live test and uses the supply voltage to do the test. I suspect that something is wrong with your meter/leads/probes, or as someone else has mentioned there are some bad connections, but by calculation alone your Zs is in spec, so dont worry about it. Write the calculated Zs on the sheet. The only other thing i can think of is if you are taking your Zs at a socket outlet built into the cooker isolator switch, if so i would testing at the cable connections and if all is well, then replace the cooker isolator. I have seen significantly out of spec results at socket outlets, and found that by connecting directly onto the cable the readings came well in. Faulty materials is not uncommon but easily overlooked because they are new.

 

[binky]

even when testing in the back of an accessory, the plating on the terminal screws can give false readings, and new items often haven't bedded in the contacts or have residual chemicals on the metal parts from the manufacturing process. I always know when this is an issue with my old Megger, it always comes witth 96.5 Ohms. Firk knows why, but that's what it does. But as SirKit said above the dead tests are perfectly acceptable anyway.

 

[Ardet R]

You might be getting a "high" resistance through the MCBs, have you tried testing with the MCB off and a Line earth link at the end of the circuit and the tester probes across in and out of the MCB?