Higher-accuracy low Ze test procedure

[IQ Electrical]

This is the method that we use to ensure a higher accuracy on very low Ze readings.

At first glance, a difference of say 0.09 Ohms does not seem much on a Ze reading but when this value is used (by the instrument or by calculation) to determine PFC then the difference can be massive.

For example: 230/0.10 = 2.3KA 230/0.01 = 23KA

It goes without saying that any connections must be made safely and live testing carried out with equipment in accordance with GS 38.



1 Find a suitable impedance to be used-this should be around about 0.5 Ohms, R2 leads are good for connectivity reasons.

2 Find an outlet on the installation (fused connection unit, socket outlet etc.) that is likely to have a Zs of around 0.20 Ohms.

3 Measure that Zs with the instrument test leads (not the BS 1363 plug lead) and note the value.

4 Measure the Zs again but with the selected impedance (SAFELY) in series with the earth test lead.

5 Subtract the value measured in step 3 from that measured in step 4 and you now have a value for your test impedance (only for that particular installation).

6 Now return to the origin and measure Ze with the test impedance in series with the earth lead.

7 Subtract the value obtained in step 5 and you now have a far more accurate figure for Ze.

Obviously ambient temperature, harmonics, transformer noise etc. all affect readings but unless you have the transformer impedance and the details of the distribution circuit length and CSA etc. then this is about as accurate a reading as you can hope to measure!

 

[Guest123]

Thread Stuck.

 

[Guest 004]

round of applause for IQ

 

[IQ Electrical]

I forgot to mention that the test can also be used to determine Prospective Short Circuit Current (L-N) by using the impedance in series with the neutral test lead.

 

[Guest 004]

I will do a calc for that, as that is all the test instrument does anyhow

 

[T&Earth]

Hi IQ
Thanks for explaining this method, very useful

T&E

 

[IQ Electrical]

Hi IQ
Thanks for explaining this method, very useful

T&E

No problem, it's quicker to do than it looks when it's written out!

 

[Guest 004]

No problem, it's quicker to do than it looks when it's written out!

yes agreed, when I first read it I though 'what the feck', but as I have done R2 readings before the procedure is quite easy to do

 

[mozzo]

Applying basic electronics, I would assume that within the meter very high ohm, low tolerance (accurate) resistors with a high wattage for safety would be used to measure a voltage drop that would be used to calculate the PSCC. The introduction of a low ohm resistor external to the test meter would surely be insignificant in the measuring process. The added external resistor would also have to be super accurate if you then use the external resistance value to subtract from the measured value. Again, for such low measurements, you would also need to measure the value of this external resistor and not juts take the reading on the packet as gospel. However I do see the logic in what you have described as if your meter is only capable of reading to 2 decimal places and the Ze initial Ze measured is near the end of the range then boosting the Ze would bring the reading within a range that is more acceptable and more accurate to your meter. Again this also depends on the meter and the accuracy of the meter in relation to the upper, lower and mid range points of measurement. If your meter is totally accuate throught its entire range then the prescribed technique is not required.

 

[Cammy]

I'm presuming that this method would be ok to use and explain for an (Elecsa) assessment? I have changed a consumer unit and the Ze is coming out at 0.03 ohms; so I have got some interesting PFC results off that. The substation being across the road from site is just great! :furious3:

 

[Mark_Burgess]

I take it this method would work for the PFC/PSCC?

Looks like I will have to try this later, the reading I got yesterday of 50kA was taken 3 times to ensure I was not reading it wrong, would be 100kA between lines!!

 

[IQ Electrical]

I take it this method would work for the PFC/PSCC?

Looks like I will have to try this later, the reading I got yesterday of 50kA was taken 3 times to ensure I was not reading it wrong, would be 100kA between lines!!

Absolutely, where is the transformer sited in relation to the point of PFC measurement?

 

[Mark_Burgess]

Half a mile or more down the road, this factory used to be a joinery and had one hell off power consumption, the incomer fuses must be in the region of 200A, I cant identify them and will need to call SSE out as my customer has decided he really wants to know what they are for the PIR, neither SSE or B,Gas know what is installed. The cable is approx 150mm CSA.

 

[Mark_Burgess]

Phew, thats better, 4.9kA!!

 

[Engineer54]

50ka is High!! What you'd expect from a 2MVA TX, even then you would need to be dammed close to that TX ...lol!!
Think you must have been missing the decimal point there...lol

 

[Mark_Burgess]

[QUOTE Think you must have been missing the decimal point there...lol[/QUOTE]

I did think that first time, so repeated the test 3 more times, even put the back light on the display to make sure!!

 

[telectrix]

never had these problems when the most advanced test equipment we had was a AVO 8. LOL.

 

[Mark_Burgess]

Managed to get it powered down today and main isolator switch opened, 250A fuses and found a cable marking of 185mm, so I was close before but not quite there.

 

[VoltzElectrical]

This is the method that we use to ensure a higher accuracy on very low Ze readings.

At first glance, a difference of say 0.09 Ohms does not seem much on a Ze reading but when this value is used (by the instrument or by calculation) to determine PFC then the difference can be massive.

For example: 230/0.10 = 2.3KA 230/0.01 = 23KA

It goes without saying that any connections must be made safely and live testing carried out with equipment in accordance with GS 38.



1 Find a suitable impedance to be used-this should be around about 0.5 Ohms, R2 leads are good for connectivity reasons.

2 Find an outlet on the installation (fused connection unit, socket outlet etc.) that is likely to have a Zs of around 0.20 Ohms.

3 Measure that Zs with the instrument test leads (not the BS 1363 plug lead) and note the value.

4 Measure the Zs again but with the selected impedance (SAFELY) in series with the earth test lead.

5 Subtract the value measured in step 3 from that measured in step 4 and you now have a value for your test impedance (only for that particular installation).

6 Now return to the origin and measure Ze with the test impedance in series with the earth lead.

7 Subtract the value obtained in step 5 and you now have a far more accurate figure for Ze.

Obviously ambient temperature, harmonics, transformer noise etc. all affect readings but unless you have the transformer impedance and the details of the distribution circuit length and CSA etc. then this is about as accurate a reading as you can hope to measure!

I feel stupid for not understanding this. Perhaps there are too many words? How do I do this with my Megger 1553? I know how to do Loop testing with it, but i don't really get what is meant here by 'safely in series with the earth test lead'.

Can anyone clear this up, it sounds interesting.

Thanks

 

[adamh]

Isnt this a live test, with the main protective bonding conductors disconnected? (so no good for an assessment) What is the extra impedance for?