Loop method using Fluke 1653

[hahol89]

Hi all!
I tried to measure earth resistance by loop method using fluke 1653 at MR system (TN) in hospital. But got abnormal results. The primary (upper) display showed 0.16 - 0.2 (loop impedance) and secondary - 0.0 - 0.1 (earth resistance)
Is there some limitations using this method? Cant find information about it.
Thank you!

 

[davesparks]

Why are you trying to measure Ra on a TN system?

 

[hahol89]

What is Ra?
Let me explain, i am responsible for power & grounding audits in my company and i got measurement procedure from my previous colleague. But i see that this is wrong way to measure earth resistance with loop method. I want to know whats is wrong? I saw information that this measurement acceptable for TT systems but not for TN. Is it true?
Thank you!

 

[davesparks]

Ra is the resistance of an earth electrode to earth, it is the value given on the second line of the display after the test you have done. This is not the best way to measure Ra, it is better to carry out a proper earth electrode test wherever possible.

However all of this is irrelevant to your TN supply, what exactly are you trying to measure. And are you a qualified electrician?

 

[hahol89]

Ra is the resistance of an earth electrode to earth, it is the value given on the second line of the display after the test you have done. This is not the best way to measure Ra, it is better to carry out a proper earth electrode test wherever possible.

However all of this is irrelevant to your TN supply, what exactly are you trying to measure. And are you a qualified electrician?

No. i am not electrician, i am an installation engineer (medical equipment). I need to measure earth resistance that our equipment used. i can carry out measurement only in our disconnet panel located neer equipment, so its impossible to use 3-pole measurement or similar methods. i think it was purpose to use fluke 1653, but i should to understand its not best way or it is wrong way?
I think the best decision it is clamp on method, for example fluke 1630. What do you think?

 

[Richard Burns]

This is the quick reference guide for measuring the earth fault loop impedance of a circuit for the fluke 1653b.

If you have RCD protection then you would need the no trip setting, if not then the high current test is better.
if you press F3 then you will move off the RE measurement and back to the fault current.

If you need more information then this is a link to the user manual.

 

[hahol89]

Richard, thank you! But i already saw this manuals. My question is limitations for this measurement. Is it acceptable for TN systems? Should i disconnect earth conductor from earthing busbar?
I found next topic and i want to understand is loop method the same with method in part 4 of this topic.
Thank you

 

[Richard Burns]

Using the Z1 method, from the Fluke 1653, of measuring the earth fault loop impedance is the de facto standard for TN systems.
The limitations are that the measurement is not very accurate at very low values of EFLI due to the limited resolution of this type of meter.
Using the no trip measurement limits the test current to about 15mA and so introduces noise on the measurement that reduces accuracy.
contact resistance on the connections can also affect the result so good connections are essential.

This method can also be used on basic TT systems for a reasonable approximation of the true value of the resistance of a an earth electrode.
It is not suitable for accurate measurement of earth electrode resistance for which a earth resistance tester would be required.

In order to gain a value for Ze (the external loop impedance) of an installation it is necessary to only measure at the origin once all bonding conductors are disconnected (or disconnecting the main earthing conductor and measuring to that) otherwise a falsely low value of EFLI might be obtained due to parallel resistance paths via the bonding.
This requirement does not obtain where you are measuring the Zs of a final circuit.

 

[hahol89]

In order to gain a value for Ze (the external loop impedance) of an installation it is necessary to only measure at the origin once all bonding conductors are disconnected (or disconnecting the main earthing conductor and measuring to that) otherwise a falsely low value of EFLI might be obtained due to parallel resistance paths via the bonding.
This requirement does not obtain where you are measuring the Zs of a final circuit.

Im sorry for stupid questions but what is final circuit?

 

[Richard Burns]

A final circuit is a circuit that directly powers an item of equipment.
i.e it is not a circuit to or between distribution boards.
e.g the circuit powering a your lighting, boiler or socket outlets.

 

[shanky887614]

Im sorry for stupid questions but what is final circuit?

its a ring final circuit. a socket circuit with two cables going back to the supply usually aranged in a ring to reduce the amount of cable needed

 

[hahol89]

Thanks all! But I will repeat my questions

But got abnormal results. The primary (upper) display showed 0.16 - 0.2 (loop impedance) and secondary - 0.0 - 0.1 (earth resistance)

I followed all steps from manual. What I made wrong?

 

[hahol89]

Thanks all! But what I made wrong that I got results (The primary (upper) display showed 0.16 - 0.2 (loop impedance) and secondary - 0.0 - 0.1 (earth resistance))? I followed all steps from manual.

 

[Richard Burns]

It is no good saying you have abnormal results and not telling us what you think is abnormal about the results.
A 0.16Ω or a 0.20Ω resistance is a reasonable figure for an Earth fault loop impedance.
The Re result is immaterial as it would only apply if you wanted to know the resistance of the earth component of the loop, however it is an acceptable value, slightly lower than I would expect, but acceptable as a value.

If you wanted to know the prospective earth fault current (PEFC) then you need to press F3 on the tester to change the display.

Therefore unless you can say what you think is wrong we cannot progress further.

 

[hahol89]

Sorry!
Customer measured earth resistanse with 3-pole method (1.6 ohms measured value) and asked us to check this results, so i tried it and got 0.16 - 0.2.

 

[davesparks]

Sorry!
Customer measured earth resistanse with 3-pole method (1.6 ohms measured value) and asked us to check this results, so i tried it and got 0.16 - 0.2.

What exactly do you mean by earth resistance?

 

[trev]

its a ring final circuit. a socket circuit with two cables going back to the supply usually aranged in a ring to reduce the amount of cable needed

So what about a lighting circuit, or one powering a motor or an AC unit? Aren't they final circuits?

 

[hahol89]

What exactly do you mean by earth resistance?

earthing conductor resistance

 

[rustynails]

earthing conductor resistance

To obtain an accurate resistance measurement of an individual conductor, such as r2, you will need to perform a continuity test. This is a dead test. You will need to disconnect one end of the conductor and measure the resistance using your Fluke set to continuity from one end of the conductor to the other. This method of measuring r2 is called the 'long lead method'. See IET Guidance Note 3 for more information on dead testing.

Everybody, who knows anything on here has given you the right advice regarding the no trip earth fault loop impedance testing with your fluke. Perhaps, you mean earth leakage...

Or perhaps this is a wind up?

 

[hahol89]

To obtain an accurate resistance measurement of an individual conductor, such as r2, you will need to perform a continuity test. This is a dead test. You will need to disconnect one end of the conductor and measure the resistance using your Fluke set to continuity from one end of the conductor to the other. This method of measuring r2 is called the 'long lead method'. See IET Guidance Note 3 for more information on dead testing.

Everybody, who knows anything on here has given you the right advice regarding the no trip earth fault loop impedance testing with your fluke. Perhaps, you mean earth leakage...

Or perhaps this is a wind up?

You absolutely right, i mean earth leakage resistance, sorry but this is difficulties of translations))