2391 Testing and Inspection practical.

[alex28]

On the 3 phase board will there be 3 Ze's? bit confused as on schedule of test results it looks like only one Ze is asked for

 

[Adam_151]

On the 3 phase board will there be 3 Ze's? bit confused as on schedule of test results it looks like only one Ze is asked for

No, the Ze is the worst case reading (highest impedance) you get after you have tested the fault loop between the means of earthing and each of the three phases in turn

Same kinda thing with the PFC, just the one number to write down, its the worst case (highest ka) out of the three P->N PSSCs and three P->E PEFCs, *doubled* (this is to take account of faults involving more than one phase)

Don't try and measure PFC phase to phase unless you are sure that the instrument is designed to measure phase->phase!... best stick with the single phase measurement and doubling just incase

 

[FingRinal]

2391 Practical is a bit like a driving test. The examiner is hoping for a easy time with a group of 'safe' and competent electricians. Start flapping about and looking nervous and you'll attract more attention. Start off in the right manner, let him see your safe isolation procedures and confidence with your test kit and you'll be off on the right foot. The examiner will start to focus elsewhere as basically he is the safety man. You might make the odd minor mistake which might 'prompt' a question from the examiner. If you are in control and recover with confidence they will be over looked.

If you look like you are out of your depth, start flapping or are too tense you might fail purely on the basis of safety. The examiner won't want to take any chances with a live board.

So, keep calm, keep logical and keep to the basics. There isn't a lot of slack time so don't hang around.

Also, don't assume that the test rigs are fault free. Some take a right hammering or the previous bloke might have left a few faults on. So, if you get a few funny readings don't assume that you are wrong as it could be the rig.

 

[Adam_151]

Also, don't assume that the test rigs are fault free. Some take a right hammering or the previous bloke might have left a few faults on. So, if you get a few funny readings don't assume that you are wrong as it could be the rig.

Or an over zealous technician with too many wirewound resisters to make the readings resemble a real installation, Zs for the ring final ended up at about 2 ohms!

 

[alex28]

thanks for the reply-so its the highest Ze out of the 3 phase-earth loop tests I write down-cheers

 

[Adam_151]

Aye,

Main switch off, earthing conductor disconnected from met, tester earth lead clipped onto earthing conductor, probe the line side of tester onto L1 on incomming side (probe neutral onto neutral as well if you tester needs one), take reading, move the line side onto L2, take reading, then L3, take reading

Then take the highest worst case one, so if I got 0.31, 0.35 anf 0.33, then I'd write down 0.35 as thats worst case (highest impedance)

Reconnect means of earthing before re-closing main switch!

Do you have a copy of GN3?

 

[alex28]

Yeah got GN3 but didn't explain the three phase situation all that clearly,so whatever reading I get for PEFC I just double the highest one and put that in the Ka box? cheers

 

[Adam_151]

Yeah got GN3 but didn't explain the three phase situation all that clearly,so whatever reading I get for PEFC I just double the highest one and put that in the Ka box? cheers

Not quite, take the highest of all the PSCC readings and the PEFC readings you get (although normally PSCC readings will be higher than PEFC ones), then double it for a fault involving mutiple phases this is what you write in the PFC box

(some 3-wire testers let you take PSCC and PEFC by just toggling a button)

PEFC - Prospective Earth Fault Current
PSSC - Prospective Short Circuit Current

PFC - Prospective fault current (for either type of fault)

 

[alex28]

thanks for that, so double the highest PSSC will be the answer-remember lecturer saying u don't have to bother with PEFC as PSSC will be higher, cheers

 

[plugwash]

remember lecturer saying u don't have to bother with PEFC as PSSC will be higher

that is bad advice, PSSC will usually be higher but it is not something you can rely on.

Also while the double it rule has a fairly substantial safety margin it is still IMO better to read the real phase-phase PFC value if your tester can do it (think of the case of a bad neutral for example).

 

[alex28]

just failed practical on two out of four assessments-only spotted 11/12 faults on the board and left key in main switch padlock when doing motor starter circuit! gutted because was going ok, got Ze and PFC right for 3 phase system and passed the other two assessments (insulation resistance and working out Zs for the MCB's using rule of thumb & formula method)