Help with phase rotation fault question please

[Robbo118]

Hi,

When proving supply and polarity at a three phase cut out on a new supply with test lamps

L1 - N is 230V L2 - N is 230V L3 - N is 230V

L1 - L2 is 400V L1 - L3 is 400V

So far so good. But here's the problem:

L2 - L3 is NO LIGHT!!!

The only thing I can think of is that L2 is fed from L3. Maybe two phases fed from one by incorrect termination at cut out.

Anyway, I just thought I'd ask it on here and see if any of you wise people agree!!! Or know of any alternative reasons.

This is for a exam question and any help would be greatly appreciated as I have no one else to ask and it's very hard to find answer on internet so far.

Thanks,

Rob

 

[Pete999]

Have you checked your voltage indicator on a proving unit properly, could be your test lamp is faulty or a poor connection with the probes. Didn't we have this self same question posted a week or 2 ago?

 

[telectrix]

we did. apparently, the voltage testing of phase rotation is a bit of a fudge. the OP in the other thread had similar problem. answer is to use a proper phase rotation meter.

 

[Pete999]

we did. apparently, the voltage testing of phase rotation is a bit of a fudge. the OP in the other thread had similar problem. answer is to use a proper phase rotation meter.

Agree,

 

[Toneyz]

What is the exact wording of the exam question as this will define the answer they want. It is like safe isolation check last test on known supply the lamp could of failed on undertaking L1-L3 test the John Drummond has a proving unit on it.

 

[Pete999]

What is the exact wording of the exam question as this will define the answer they want. It is like safe isolation check last test on known supply the lamp could of failed on undertaking L1-L3 test the John Drummond has a proving unit on it.

he may not have a JD voltage indicator

 

[telectrix]

is it my old eyes, pete, or did you just mention JD?

edit: just spotted, it's a mark on my screen, makes the I look like a J.

 

[Robbo118]

Well I did think it could be the test equipment (lamps) going faulty... but as it's an exam question I wasn't sure if that's the answer their looking for. The question is exactly how I wrote it in my initial post. They definitely want you to be using test lamps as phase rotation tester isn't mentioned. It seems they are asking me to check the voltage on a new installed three phase cut out from DNO.
So all I can think of is either faulty test lamps, which is why we always test before and after.
Or somehow L2 is looped to feed L3 and that reason for results. Everything else is ok it's just No Light between L2 and L3 when we should be expecting 400V between phases.

I'll have a look and see if I can dig this post up from 2 weeks ago... probably someone else doing the same exam lol.
Thx

 

[telectrix]

Phase rotation advice - http://www.electriciansforums.co.uk/threads/phase-rotation-advice.122634/page-2#post-1295663

 

[Pete999]

is it my old eyes, pete, or did you just mention JD?

edit: just spotted, it's a mark on my screen, makes the I look like a J.

John Drumond Tel not Jack Daniels

 

[Toneyz]

Well I did think it could be the test equipment (lamps) going faulty... but as it's an exam question I wasn't sure if that's the answer their looking for. The question is exactly how I wrote it in my initial post.
Well I've never seen a exam question wording like that.

 

[Richard Burns]

I would agree with the OP's analysis.
To have 230V to Neutral in all cases means that neutral is connected to earth and each phase has a phase connected.
To have no potential difference between two of the phases means they must be at the same potential i.e. fed from the same phase.
So either incoming L2 is disconnected and incoming L3 connected to L2 and L3 or incoming L3 is disconnected and incoming L2 connected to L2 and L3.
Not a common fault I would have thought.

 

[Robbo118]

Thanks. I like some of the wording you used there... all examiners or electricians love it when you start talking about 'potential difference'... it's the science side of it all which I enjoy! So I think I will throw that in there haha.

Yeah so if each phase to neutral were 230V then that suggests they are all connected.
It's just the potential difference between L2 and L3 that's the concern. And like I said the only thing I can think of is that L2 must be looped to feeding L3.
Cheers

 

[Krispy_88]

Actually had this exact same issue on a job a few weeks ago at a large pub we service. Original call was their AC not working and Kitchen Extract System failing. Found that i was getting nothing between L2 & L3 at the mains incomer to the whole place so called UKPN to find that they had messed up while carrying out some works locally . Course it only really effected their three phase equipment.

 

[Richard Burns]

Thanks. I like some of the wording you used there... all examiners or electricians love it when you start talking about 'potential difference'... it's the science side of it all which I enjoy! So I think I will throw that in there haha.

Yeah so if each phase to neutral were 230V then that suggests they are all connected.
It's just the potential difference between L2 and L3 that's the concern. And like I said the only thing I can think of is that L2 must be looped to feeding L3.
Cheers

Careful not to use wording you would not normally use in an answer or the examiners may think you have cribbed the answer off an electricians forum!
I think the best you can say is that L2 and L3 are sharing a supply phase and that phase is not L1, you would have the same results if they were sharing the incoming L3 or L2; but a nice big bang if they were sharing both!

 

[Robbo118]

Lol good point. Ahhh yes I see. We don't know which phase is looped to feed the other one. That's a good way of wording it that L2 and L3 are sharing a supply phase and that it's not L1.
Thanks for your help.