High zs readings low ze readings

[Badged01]

Re: High zs readings low ez readings

Jesus Christ almighty, you don't understand the basics of why we carry these tests out?

I most certainly do, sure you do? Have a read of BS7671/GN3 for approved/correct testing methods, you might be suprised .......

A Zs test and an R1+R2 test are different things entirely. The result almost never equals the calculated value.

Never said they were the same, the discussion is around "live" v "dead" Zs and how you can calculate the value using Ze and (R1+R2). And no they (Zs) dont equal the same value for a variety of reasons, which is another reason why the the "dead" (R1+R2) should be the method of choice.

A Zs test passes a relatively high current through the circuit which will identify any poor connections far better than the low current continuity test.

But the level of difference will be low in the overall picture. Try doing some controlled testing of electrical joint resistances at various level of connectivity. Aside from almost loose you will again be suprised ......

The Zs test will give you the impedance including all paralell paths so a circuit tested this way may pass when the calculation method fails it.

Correct point, exactly what I taught students but not a reason to justify "live" testing. If you have parallel paths then your "live" Zs reading should be lower because of it. Hence with the (R1+R2) test you are getting a worst case scenario which is what you want to know should any of these parallel paths fail. You do the same for the Ze value, remove parallel paths before testing.

The Zs test will include all of the terminations in the CU and the protective devices, highlighting any faults which could start one of these CU fires we are hearing so much about.

The skill of the tester can make his connections so that he includes these, terminations, resistances thro' circuit breakers etc. Think about it, Im sure you can fathom out how to do it ......


"Electricity kills, 'dead' testing should be the method of choice"

 

[PEG]

Re: High zs readings low ez readings

...and, as the circle is formed,in the playground,the teachers sense trouble....and confiscate the remaining MFTs'...

 

[markythesparky]

Another thing we have come across is hager rcd's, they generally add about 0.65 ohms to your zs reading

Never come across that, especially with Hager. In any case I'm not sure it would ever be acceptable or even possible for any RCD/RCBO to increase earth loop. How it would even do that as RCD's monitor an the balance between phase and neutral and don't monitor the connection to earth. Yes some have a functional Earth but it is not part of the CPC of the circuit so how would the Zs be affected?

 

[Leesparkykent]

Never come across that, especially with Hager. In any case I'm not sure it would ever be acceptable or even possible for any RCD/RCBO to increase earth loop. How it would even do that as RCD's monitor an the balance between phase and neutral and don't monitor the connection to earth. Yes some have a functional Earth but it is not part of the CPC of the circuit so how would the Zs be affected?

Of course it could, think about it...

 

[Guest77]

I am absolutely dismayed at some of the replies here, the trade is now on it's knees, shocking.

 

[markythesparky]

Are you thinking a high resistance on the phase?

 

[Leesparkykent]

Are you thinking a high resistance on the phase?

No how the RCD can increase the EFLI reading by resistance on the line through the RCD.

 

[markythesparky]

Yes. And to repeat, the RCD won't increase earth loop through the CPC. Im thinking there is a possibility of it through phase resistance as the phase conductor is connected to the RCD, the earth isn't!

 

[Leesparkykent]

Yes. And to repeat, the RCD won't increase earth loop through the CPC. Im thinking there is a possibility of it through phase resistance as the phase conductor is connected to the RCD, the earth isn't!

yeah, hence it can increase the earth loop reading. Easy way to check would be to measure the voltage either side of the RCD.

 

[markythesparky]

I am absolutely dismayed at some of the replies here, the trade is now on it's knees, shocking.

Time of the month?

 

[davesparks]

Never come across that, especially with Hager. In any case I'm not sure it would ever be acceptable or even possible for any RCD/RCBO to increase earth loop. How it would even do that as RCD's monitor an the balance between phase and neutral and don't monitor the connection to earth. Yes some have a functional Earth but it is not part of the CPC of the circuit so how would the Zs be affected?

The earth loop impedance incorporates the impedance of the line, cpc, earthing conductor, supply cable etc. The RCD is connected in the line conductor and so has an effect on the earth loop impedance