Some opinions required

[pompeygit]

Am currently studying 2391 and wonder if someone could advise me on some things that are confusing me.
1) In Hinsleys book, Testing Electrical Installations, he quotes some examples of PSCC he talks of measurement or calculation. I quote ' Assume you have a reading of 0.10 ohms, you need to divide the voltage at this point by the resistance, e.g. 230v/0.01 ohms = 23kA'. My question Is he correct to use Uo 230 rather than Uoc the transformer voltage which could be 240 or more at time of measurement?
2) When measuring PSCC are the only relevant points to be measured at the origin of supply and origin of a circuit(at the breaker)?
3) When testing Zs on 3 phase motors Hinsley says that MEB should be disconnected......surely that can't be right?
4) Also when doing Zs on a small induction motor Hinsley says remove the star/delta links and test at the motor. We are being taught that the circuit should be broken down into sections and measured and then a calculation made . How would you personally measure Zs on a motor?

 

[Spark123]

1) I'd use Uoc for fault calculations as that is the potential difference pushing the current around the system, the fault path includes the impedance of the supply cables etc.
2) Yeah, basically where there is a fuse or circuit breaker which needs to operate under fault condition. (Remember - you need to do PEFC and PSCC, the larger value being PFC)
3) Definately wrong!! You should never have a MEB disconnected with the supply energized!
4) You can do the calculation in small bits if you like i.e. pre contactor cabling, post contactor cabling + Ze.
You can remove the links from the motor, this will stop it running if you are wanting to do a live efli test.

 

[ricicle]

You can remove the links from the motor, this will stop it running if you are wanting to do a live efli test.

Extreme caution required though - even for the most experienced of testers

 

[RF Lighting]

Could you put the earth clip onto the motor, and the live prong onto the incommer terminals of the local isolator / starter etc?

 

[pompeygit]

Thank you for your input fellas

 

[ban-all-sheds]

3) When testing Zs on 3 phase motors Hinsley says that MEB should be disconnected......surely that can't be right?

3) Definately wrong!! You should never have a MEB disconnected with the supply energized!

It wasn't in the context of 3Ph motors, but wasn't there a long debate on the IET forum a couple of years ago about Mr Hinsley's advice on disconnecting earthing and bonding during installation testing?

 

[Spark123]

I'm not sure, I don't read the IET forum much tbh.
In the context of C&G2391 I'd imagine you'd be shown the door for disconnecting the MEB on the test rig and powering it up.