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?