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Discuss Higher-accuracy low Ze test procedure in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

[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!!
 
never had these problems when the most advanced test equipment we had was a AVO 8. LOL.
 
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.
 
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
 
Isnt this a live test, with the main protective bonding conductors disconnected? (so no good for an assessment) What is the extra impedance for?
 
Pardon my ignorance, but...! Isn't this method merely describing an alternative to 'nulling' the meter with the test leads attached? Or are you saying that the apparent resistance of the (R2) test lead would vary with different installations, depending on the Ze????
 
It's to do with how the accuracy of the meter becomes more inaccurate, or inconsistent, when the impedances are very low. And as IQ has stated the resulting calculated/measured fault currents vary greatly. By applying a controlled resistance into the circuit, the meter can perform more accurately/consistently.
 
If you have a calibration check box could you use one of the resistors there? They are meant to be accurate & consistent and have good connection points
 
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!

Why dont you just test it normally like everybody else I don't understand the logic behind this most readings on TNS supplies are about 0.2 to 0.3 ohms and PME is a little lower, Tower blocks tend to be a lower if you test them in the flat because of the bonding and you don't get access to the main intake but in general it's an easy test why complicate it
 
Why dont you just test it normally like everybody else I don't understand the logic behind this most readings on TNS supplies are about 0.2 to 0.3 ohms and PME is a little lower, Tower blocks tend to be a lower if you test them in the flat because of the bonding and you don't get access to the main intake but in general it's an easy test why complicate it


The reason
Widdler quote
It's to do with how the accuracy of the meter becomes more inaccurate, or inconsistent, when the impedances are very low. And as IQ has stated the resulting calculated/measured fault currents vary greatly. By applying a controlled resistance into the circuit, the meter can perform more accurately/consistently.

The outcome
The level of Pfc measured could mean the difference between a rated device passing or failing,with readings that are low he demonstrates the large variation that can occur
I thought it was an excellent and informative post by a guy who definitely knows his onions

 
The trouble with this method, at such low readings (instrument resolution of 0.01 Ohm, looking a measurement of 0.0x Ohms), is that it doesn't eliminate the instrument's 'number of digits or counts' error.
So, it's essentially a futile exercise!
 
The reason
Widdler quote
It's to do with how the accuracy of the meter becomes more inaccurate, or inconsistent, when the impedances are very low. And as IQ has stated the resulting calculated/measured fault currents vary greatly. By applying a controlled resistance into the circuit, the meter can perform more accurately/consistently.

The outcome
The level of Pfc measured could mean the difference between a rated device passing or failing,with readings that are low he demonstrates the large variation that can occur
I thought it was an excellent and informative post by a guy who definitely knows his onions

There is only oe approved method of measuring Ze my advice is stick to the industry guidelines in GN3
 
There is only oe approved method of measuring Ze my advice is stick to the industry guidelines in GN3

Yes, but if the DNO's TX is basically in the garden, your MFT Ze/PFC values obtained will be next to useless, so unless you know the TX size and it's impedance along with other details to calculate these values your snookered. Then again, you could always use a very expensive high resolution (0.00X) ELI tester!! IQ's method will give you a higher level of accuracy applying the use of a known/controlled resistance into the circuit being measured....

Oh but i forgot, the picture book GN3 doesn't actually mention anything about thinking outside of the box when needs arise does it!!
 
I dont see this technique helping.

Lets say the Impedance is 0.02 ohms, at 230 V this equates to 11.5kA.

Now if we introduce an impedance of 0.5 ohms, our circuit is now 0.502 ohms.

At 0.502 ohms standard meters will have a 10% +/- and a 5 digit error +/- around the 0.5 area.

So this equates approx to a +/- of 0.1 ohm, so even with the 0.5 ohm added the reading could be 0.6 ohms or 0.4, a differnace of 0.2 ohm's.

Cheers
 
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