testing and inspection

[jeffy]

hey, im attending a inspection and testing course next month, i do not do any domestic really as im more industrial but trying to study as much as i can beforehand so i have the best shot. at the moment im really confused as im reading about ze tests and as i understood it you do the main test at the fuse board which gives you ze for the installation which you add to r1+r2 for zs. how come you do ze tests on each circuit as well like when you plug your earth fault loop impedance tester into a socket? would this give you a seperate ze to the main one you tested? im sorry if i havent expalined this very well but im very confused lol.

thanks for your help guys

Jeff

 

[telectrix]

your Ze is the external EFLI. measured at the DB with the main earthing conductor disconnected to avoid parallel paths. fot a TNC-S the max. Ze is 0.35. for TN-S it's 0.8. for a TT earthing arrangement it's any thing up to 100 as long as it's stable. you measure Zs for each circuit, as these readings depend on the resistance of the cable of each circuit (R1+R2). added to Ze to give the total EFLI for the circuit/s.

 

[stef]

You do Ze test only once at the source of the supply with the earthing arrangements disconnected. Zs tests are done from point to point, taking the highest reading.

 

[oldtimer]

A Ze test is to confirm that the supply has a seperate earth hence you disconnected all earths and then test it. Now for example if you did not do this and just tested it you may have a borrowed earth let me explain

you live in a 1st floor flat but have no main earth yet when you test the Zs at say a socket you get a good reading ie Ze + r1=r2 =Zs but because your gas and water pipes have been earthed and so is the guy below you and because the gas and water and gas supply pipework is common to you both because they are metal so you in a sense are borrowing a earth from him or in other words a parallel path.

One other point this could be the same say for a semi so you think no problem their then well no so you are borrowing via earth from next door and you get a letter saying we will be in your area to renew the gas and water supply pipes to your property but here is the catch they replace the metal pipework with plastic ie the pipework is no longer electrically and mechanically sound or in other words you have just lost your earth and this is becoming more common so now you know the importance of checking the Ze of a property.

Just to add when they do change the pipework they put a card through your door saying get your earth checked out but 99.9% of owners dont bother but it is being highlighted by of all people British Gas who come in to maintain the boiler and they check the Zs with a Socket&See led pass/fail tester and thats when I get a call