Earth Faulth Loop Impedance HELP!!!

[martinh89]

Hi can somebody shed some light upon correction factors for earth fault loop impedance. I know that the vales of zs in bs 7671 are if the conductors are running at max temp ie worst case scenario , am I right in believing that if you get a measured result you apply 0.8 to it?

 

[martinh89]

Also ive got a question im struggling with . ........If an earth fault loop impedance tester measures a value of 0.55 ohms at 20 degrees c, what is the corrected value (using the rule of thumb method) to allow for conductor maximum operating temperature. its gives the answer 0.69 but if I use the rule of thumb method I do 0.55x0.8=0.44 ????????? just a little confused

 

[i=p/u]

1.2 try..

 

[martinh89]

Thanks but why am i useing it

 

[i=p/u]

The book gives you readings @ 20 degrees or something so you multiply by 0.8 and check your measurement

32A =1.44 x0.8 =1.15

 

[martinh89]

Yea i understand stand that, but what about when you need to find the maximum zs whilst at maximum operating temperature?

 

[Pete999]

The BGB is a design document, check your Zs from the BGB against the same device in in the OSG

 

[Richard Burns]

You are measuring a resistance when you measure Zs, think about how resistance works:
When the conductor gets hotter the resistance increases.

When you measure Zs the conductors are at ambient temperature, if they were at 70°C the resistance would be higher.

Say you have a measured value of 0.5 then the resistance will be greater than 0.5 at 70°C therefore multiply by 1.2.

If you have a Zs value of 0.6 in BS7671 then the resistance when measured could be lower (so that on heating the resistance rises to 0.6) therefore multiply the 0.6 by 0.8 (the rule of thumb).
1 / 0.8 is approx 1.2.

 

[telectrix]

Also ive got a question im struggling with . ........If an earth fault loop impedance tester measures a value of 0.55 ohms at 20 degrees c, what is the corrected value (using the rule of thumb method) to allow for conductor maximum operating temperature. its gives the answer 0.69 but if I use the rule of thumb method I do 0.55x0.8=0.44 ????????? just a little confused

because the resistance increases with temperature rise, you divide by 0.8, not multiply. to correct from 20deg. to 70deg.

 

[Farmelectrics]

Use your gn3 or on site guide for your zs values they have your 80% values already applied your bgb are the 100% values

 

[martinh89]

Thanks for all the replys i think im beginning to get it, @farmelectrics i know therd in the osg but i wanted to know why and how i got them results so cheers for the help guys

 

[Engineer54]

Can't say i've ever used the 0.8 correction factor, except within areas that has typically significantly higher working ambient temperatures (few and far between too). Like when is a conductor ever going to even approach 70'c, let alone be normally working at that sort temperature??


But then again, the vast majority of our circuit Zs values will be more than adequate to accommodate applying the 0.8 correction factor anyway!!

 

[SirKit Breaker]

The regs are a design document, and most sparks day to day will leave it to gather dust in the van. The Zs tables contained are at 70 degrees, so you apply the 0.8 correction factor (if you want the science and calcs it is in Appx 14).

Most sparks will refer to the OSG, which as someone has already said, the tables have been corrected for us, and are at ambient (20 degrees) so that is the figure you need to meet.

You could go on and on, but to be fair a slight rise or drop in ambient temp is not going to make any significant difference to your Zs values, so the ones in the OSG are adequate for most day to day scenarios you will encounter, hence we use the OSG and not the regs. We may on occasion get colder, but seldom do we experience temperatures of more than 30 degrees unless it is a specific installation that does get hot, saunas etc.

Cheers…………Howard

 

[telectrix]

but, as eng54 pointed out, how often do you get cables running anywhere near 70deg.? esp. in a domestic setting. this would only happen if the cable was undersized, or on 1 of them god-awful megawatt showers.

 

[i=p/u]

Uh done a petrol station this size of a garage to park a car and it had 56 circuits.. Over kill