Completely confused by fault in new installation

[slide]

So I've just completed (well apart from this issue) an installation for a coffee shop/cafe. It's all tubed out in galv conduit, as the client wanted the 'industrial' look.

The circuit thats being problematic is the lighting. I have created pendants from thru boxes on the ceiling, which drop to metal E27 holders, via 0.75 3 -core flex. All holders have correct polarity and really good r2. All meggerred A:OK between all lines and conduit etc

So take the bank of 8 pendants in the window for example. Disconnect switched live from light switch over at the grid, with no lamps in, L-E is open circuit. Put your first lamp in and its 150ohms, stick another lamp in and it reduces to about 75 - you keep adding lamps until eventually the circuit has a low enough resistance to throw the breaker.

I nit picked through it for hours today, with a rather worried client looking on as he needs to open (the woes of commercial work!)

Could it be related to the fact that at the origin of the supply, there is a reading of about 0.25 between neutral and earth. Putting a lamp in makes a pathway between L and N so therefore to earth as well?

I need to look at it with a fresh head on Monday really, but thought i'd see what others think.

Cheers

 

[Sintra]

Disconnect all conductors of the circuit at the db then close all switches. Get readings for R1 R2 and Rn and compare. N & E will be connected at source and adding lamps will give you a resistance between L & N which will reduce the more lamps you add. You haven't stated number and wattage of lamps or size of OCPD in this circuit.

 

[trev]

Could it be related to the fact that at the origin of the supply, there is a reading of about 0.25 between neutral and earth.

You seem surprised by this, are you talking about 0.25 ohms here?
Think about it mate, if this was the cause of your problem it would surely be affecting all circuits.

 

[FAA Greenie]

I may have mis read this, but is there an actual problem here?

Have you had the circuit live yet? And does the breaker actually operate?

If you are fitting lamps in parallel then the resistance is obviously going to reduce.

 

[Meggerman]

Blonde moment. Too many beers

 

[Simonslimline]

If your lamps really are 150 Ohms each then your lamps are about 350W each. You state there is 8 of them. So you have 2.8Kw of load there. Over 12A.

Please correct me if i am misunderstanding this but isn't the 150ohms the cold resistance of the lamp?

 

[showerman]

You say that as you put more lamps in circuit then eventually the breaker trips , are other
circuits working ok while you are carrying out this or is board totally off , borrowed neutral
if dual rcds perhaps ? Scuffed neutral in conduit tracking to earth when enough current flow ?
( similar type of fault when you catch neutral with fixing screw on socket, ok until current flows
then rcd operates ) Best of luck , Ken .

 

[Meggerman]

Please correct me if i am misunderstanding this but isn't the 150ohms the cold resistance of the lamp?

Too many beers and half watching TV, of course, disregard

 

[davesparks]

The way I'm reading it he is adding lamps and testing L to E continuity after each one.
MFTs test with enough current to trip an RCD if the resistance is low enough.
So after a certain number of lamps the RCD operates when the test current is applied.

 

[Lucien Nunes]

Could it be related to the fact that at the origin of the supply, there is a reading of about 0.25 between neutral and earth.

This bit worries me. Of course there is a low resistance between neutral and earth, that's what makes neutral neutral. But why would you be doing any kind of insulation test (because that is what your multimeter test between L & E is effectively) with the circuit neutral connected to the supply? One possible reason is habit, if you are doing a global test and don't often work with TP+N boards, forgetting that the isolator does not normally isolate the neutral as the DP isolator in single-phase boards does. But as other posters have pointed out, the circuit should be completely isolated in itself.

If it is, and I've misunderstood your reference, then your earlier test results are wrong or you have a fault between circuits like a borrowed neutral. Perhaps when you did the first IR test, neither circuit was connected to the supply and they both tested good albeit interconnected. Now one circuit is energised, the other circuit that is supposed to be isolated reads faulty.

But what breaker trips, or do you merely expect it to trip? It's not clear in your description. Please confirm that both lives of the circuit under test were isolated during the test...

 

[FAA Greenie]

How did it work out then OP? Any news?