Zs too high at distribution boards

[Deleted member 26818]

No, had some Californian Sunshine years ago which I found to be very good.

 

[NVQ Assesor]

Hi Knasher
Max Zs for 63amp 3871 mcb is 0.76 Ohms. with your Ze at 0.08 even if you uprated your main earth conductor too 16mm it wont make a differance to the high readings you have on some of the Diss board's. Check the connections off the Glands if it is on SWA's, corrosion on the armouring will draw the readings down. Use a P-Nut lock nut with a suitible supplementary to the earth bar on both ends will help.

 

[IQ Electrical]

It's a BS 3871 type 2 60A !

I make it a maximum of 0.55 Ohms tabulated, 0.44 Ohms at 80% value????

It's so important to get these figures right as the o.p. could (rightly or wrongly) use them in his diagnosis!

 

[knasher]

Found a bit more info from PIR guys and it includes: absence of supplementary bonding in places (been given code 1, which seems a bit bit extreme - wouldn't it be code 2?). Also, mentions poor connections on some ring final circuit patresses in certain offices. Another point mentioned is main bonding being terminated at sub distribution boards and not MET. All of these things could affect Zs couldn't they? Excuse my ignorance.

One final question for IQ electrical, can you tell me how you got your Zs figures - i've looked in GN3, OS guide and BS7671, applied rule of thumb etc but am none the wiser. Can you enlighten me. Cheers again everyone!

 

[malcolmsanford]

View attachment Zs values for older protection devices.pdf

Try here

 

[IQ Electrical]

Yes, the type 2 trips at up to 7 times it's rated In current so from Ohms law, you get 230/420

That's 0.55 Ohms, 80% value of 0.44 Ohms

 

[IQ Electrical]

Deleted double post

 

[IQ Electrical]

Found a bit more info from PIR guys and it includes: absence of supplementary bonding in places (been given code 1, which seems a bit bit extreme - wouldn't it be code 2?). Also, mentions poor connections on some ring final circuit patresses in certain offices. Another point mentioned is main bonding being terminated at sub distribution boards and not MET. All of these things could affect Zs couldn't they? Excuse my ignorance.

One final question for IQ electrical, can you tell me how you got your Zs figures - i've looked in GN3, OS guide and BS7671, applied rule of thumb etc but am none the wiser. Can you enlighten me. Cheers again everyone!

Code 1 for absence of supplementary bonding?
What were the locations lacking supplementary?

Connections to back boxes are imperative with conduit/trunking being used as the sole cpc-is this the case?
That would be a code 2 on my report.

All of those points can have an effect on Zs however in my experience , it's most likely down to the length of the distribution circuit or undersized cpc in the distribution circuit.

 

[telectrix]

might be clutching at straws here, but, occasionally, you might fing a high resistance between the earth bar and the metal box in which it is mounted. if there is no fly lead between them.

 

[knasher]

It dont say in PIR where supplementary bonding is lacking - and its a pretty big building to explore, but still can do. As re. checking connections at back boxes, again thats easy enough. As re. length of circuit too long or under sized cpc, whats the most simple solution? install bigger cpc do you reckon?
Finally, thanks for shedding light on how you got Zs values for 3871 type fuses - didnt have a clue thats how you got figures! Cheers malcolmsamford too (havent checked out link yet but will do).
Is there any logical sequence to approaching problems like this or is it down to years of experience?

 

[malcolmsanford]

The easiest solution really is to fit RCD/RCBO but as we all said investigation should be in order. Im my experience if this is an older installation with all the entails it may be the original design called for a type 1 or B MCB and over the years for various reasons other types have been fitted.

 

[IQ Electrical]

If you let me know the distribution circuit cable details (CSA, approximate length, is the swa alone being used as the cpc) then I can calculate the R1+R2 and we can then see if there is a fault.

The R1+R2 added to your Ze of 0.08 Ohms will give us a worst case scenario, with actual parallel paths, your measured Zs should be lower than my calculated figure.

 

[kmcgold]

If you let me know the distribution circuit cable details (CSA, approximate length, is the swa alone being used as the cpc) then I can calculate the R1+R2 and we can then see if there is a fault.

The R1+R2 added to your Ze of 0.08 Ohms will give us a worst case scenario, with actual parallel paths, your measured Zs should be lower than my calculated figure.

good post.

 

[Dave_]

yeah do a calc..... best way...