Earth fault Loop Impedance Question

[johnboy1]

Hi All,

Q: If an earth fault loop impedance instrument records a valve of 0.55Ω at 20°c, what is the corrected valve ( Using the rule of thumb method ) to allow for conductor maximum operating temperature?

Any replies will be appreciated.

 

[m4tty]

Think you take the max Zs from BRB and times by 80% and if your efli is below this then its ok. I await being corrected tho. HTH

 

[acat]

Im with you on that one you use tables 41.2 to 41.6 of BRB




Chris

 

[zupos40]

Im with you on that one you use tables 41.2 to 41.6 of BRB



Chris

I use the tables in back of Guidance note 3 for direct comparsion of measured Zs when test temp is 10 to 20 celcuis for a more accurate comparsion.

Guidance Note 3 also has worked examples to assist you.

 

[oldtimer]

Am I right in saying that the onsite guide and GN3 has the 80% factor built into their tables. The reason I asked was that was the building I worked in had its 5 yearly PIR and the computer generated test sheets gave the reading plus the max reading from the Big Red Book and when I "enquired" about the 80% I was told no you compare with the BRB ??

 

[xraytek]

Am I right in saying that the onsite guide and GN3 has the 80% factor built into their tables??

I dont think its the 80% rule of thumb figure, instead I think the figure is adjusted for the cables nominal operating temperature. Which if I remember correctly is slightly higher than the rule of thumb figure.

 

[uksel]

it's the 80% figure in GN3 and the OSG

 

[Risteard]

I presume the question is asking what the measured value should be corrected to, which means multiply (R1+R2) by 1.2 and add this to the Ze. Mention of "Rule of Thumb" in the question is, however, incorrect. This refers to multiplying tabulated values by 0.8.

 

[cornburn]

As far as I know you don't "correct" the value, you put down the actual value. The 80% rule is applied to the maximum permissable resistance (or impedance), therefore ensuring that if the temperature of the cable rose from ambient temperature (30 degrees) to highest temperature (70 degrees), earth fault loop impedance would still be under the acceptable limit in those conditions.

That's what I was told anyway

 

[Risteard]

As far as I know you don't "correct" the value, you put down the actual value. The 80% rule is applied to the maximum permissable resistance (or impedance), therefore ensuring that if the temperature of the cable rose from ambient temperature (30 degrees) to highest temperature (70 degrees), earth fault loop impedance would still be under the acceptable limit in those conditions.

That's what I was told anyway

You correct it for comparison purposes, but you record the measured (not corrected) value.

 

[NoSparks]

Agree, record measured value and use corrected value for comparison purposes only.

 

[cornburn]

You correct it for comparison purposes, but you record the measured (not corrected) value.

Yes, that's basically what I was saying

 

[Spazz]

I know we have to take the 80% rule into account with Ze, but what is the reason? This has never been explained - is it to do with the max operating temperature of the cable?

 

[oldtimer]

Nicholas do you mean 80% of the Zs ? The reason for this is instead of doing calculations if you get 80% or less then it is a rule of thumb

 

[Spazz]

Yes the Zs sorry.
I know its a rule of thumb but what is the reason for it?

 

[oldtimer]

In my view its a fudge factor because if they held you to the regs you would have to show your calculations so to me this is an average reading thats says it allows 20%

 

[Richard Burns]

The values of max Zs in the BGB are at the conductors normal operating temperature.
when we measure values for Zs we are measuring at ambient temperature because the circuit is not energised.
As temperature rises the resistance rises therefore the limits in the BGB will be higher than we measure.
e.g if we measure 0.1 ohm at ambient temperature then with the rise in resistance as the cable reaches operating temperature would increase this to 0.125ohms. Conversely if the maximum resistance allowed is 0.125 ohms at normal operating temperature if we were to measure at ambient we would need a value of 0.1ohms (80%) to ensure appropriate disconnection at normal operating temperature.

Applying the 80% rule of thumb gives an approximation of the value you would obtain at ambient temperature.

 

[Deleted member 26818]

Am I right in saying that the onsite guide and GN3 has the 80% factor built into their tables. The reason I asked was that was the building I worked in had its 5 yearly PIR and the computer generated test sheets gave the reading plus the max reading from the Big Red Book and when I "enquired" about the 80% I was told no you compare with the BRB ??

Many of the scheme's schedules of test results ask for the maximum allowed in BS7671.
I don't know why, as it's not something requested in the BS7671 model forms, and can in some cases cause confusion.
You measure the Zs of a circuit, and the measured value is less than the maximum permitted, then fail the circuit, because the adjusted value is greater than the maximum permitted.
You then try to explain to the client why the circuit failed. No wonder the public think we're rip off artists.

 

[rich.250]

I see what you mean spin....
Sometimes makes us look to be the bad ones.

At the end of the day we are looking to ensure the cct is going to remain within the parameters of BS7671 In the worst case scenario. I.e max operating conductor temp.

And as said above, if the temperature increases so does the resistance.
When you measure a Zs there is no loading on the cct. This means no heat.

So allowing this into the situation is very important.
We need to know that if the conductor is at max temperature, we still have a big enough CPC conductor with low enough values to operate the OCPD.

And the 80% rule of thumb is the best way for us to do this.

 

[Engineer54]

I guess we should then, also consider that your test instruments are calibrated in a controlled environment at 20C. I wonder how many are as accurate at 30C and upwards??

We have to draw the line somewhere, and go by the information given to you in the various regulatory publications!! ...lol!!

 

[oldtimer]

I guess we should then, also consider that your test instruments are calibrated in a controlled environment at 20C. I wonder how many are as accurate at 30C and upwards??

We have to draw the line somewhere, and go by the information given to you in the various regulatory publications!! ...lol!!

Yep I agree I seen a national companies forms and they highlighted the max Zs then the actual Zs and some were just below the max ie 90-95% and when I queeried it I was told its in the BRB

 

[rich.250]

I guess we should then, also consider that your test instruments are calibrated in a controlled environment at 20C. I wonder how many are as accurate at 30C and upwards??

We have to draw the line somewhere, and go by the information given to you in the various regulatory publications!! ...lol!!

I'll be honest I have never even thought of the calibration temp of test equipment......
Yes we could take this too far lol.
Aren't we just verifying the design which has the temp increase allowed for.

I also don't see the need for the box on the form if it's not the value you are working too lol

 

[jdd]

80% is based @ 20c. As long as it's below this then at 70c it will be ok. (Obviously as heat increases so does resistance.)

 

[NoSparks]

And a little bit of reality:

If your close to the limit, re-connect your test meter, ie if using a plug pull the plug and re-insert, if using probes press somewhat harder. That will resolve 60% of problems. If its a double socket try the other, that will resolve another 35%. If your still close to the limit then check the voltage drop. If Zs is close to its limit then 10 to 1 on the voltage drop will be out of range, so you can forget Zs and concentrate on the real problem.

Again in my (limited) experiance if the circuit is healthy then Zs will be well be low the limit. If it's not healthy then Zs will be sky high.

What have other people found?

 

[Guitarist]

If you are having problems with a high "measured" EFLI, then try doing it with Ze+(R1+R2). This is an acceptable way of carrying out the test (the best way imo) and without the resistance of the RCD/MCB you will usually find that it is lower and probably within limits.