Discuss Earth fault Loop Impedance Question in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

J

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




Chris
 
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.
 
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 ??
 
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.
 
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.
 
Last edited:
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 :)
 
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.
 
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?
 
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
 
Yes the Zs sorry.
I know its a rule of thumb but what is the reason for it?
 
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%
 
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.
 
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.
 
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.
 
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!!
 

Reply to Earth fault Loop Impedance Question in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock