what a day, funny test results....

[matt.leung]

today i finished off some work for a client, and when it came to testing, oh boy what a head spinner it was..

hall socket, for safe isolation 2 MCB's had to be turned off... and it gave me a Zs of 129ohms

main Ze = 2.20 ohms

PFC = 110A.

at first it looked like a TNS, but now thinking that it is not, and that the house is earth via the gas and water pipes, as i could not trace the earthing back...all hidden behind boards....

also the client asked me to check some other circuits.....

Kitchen socket =

L to N = 247v

L to CPC = 139v

N to CPC = 103v

FCU =

L to N = 240v

L to CPC = 139v

N to CPC = 103v

cooker circuit was wired as a 2.5mm ring. - thank god it was a gas cooker

Zs = 2.07 ohms

voltage between the conductors all ok here this time...

out door socket =

Zs= 167 ohms

voltage between the conductors all ok here this time...

RCD trip times all ok.

dimmer switch in the kitchen... this makes no sense to me at all.....headbangheadbangheadbang

ON OFF

Cable to light

L - N 13v 247v

L - CPC 54v 126v

N - CPC 50v 83v

Supply to switch

L- N 9v 240v

L - CPC 51v 95v

N - CPC 55v 127v

can you please help me, and understand all these results..... :C :C:C

 

[milo]

sounds like a tt system and you have no earth

 

[matt.leung]

but why would i get volage between N - CPC etc....

oh yeah, the ext hood, - replace the FCU with a chrome one, took the old fuse out only to find foil, replaced it with 3a fuse, that blew, tried another 3a fuse, blew, tried 13a fuse and yes blew again...

 

[batty]

Sounds like you have serious problems ahead sorting that out.

 

[matt.leung]

any pointers..???

 

[Adrian]

do your insulation tests first these should show any problems

i don't understand why you are testing the voltages because these may not be actual true voltages and just induced votages

 

[matt.leung]

but it shows any Polarity issues.... and stray voltage, as the client has reported a couple of small electric shocks...

 

[green-hornet]

Strip it all back from the cu and start a full test on each circuit.

Main thing to do is identify the earthing arrangement you can only take all readings when you know you have a suitable and stable earth.

Do all continuity tests then insulation tests, this will narrow down any problems to individual circuits.

 

[green-hornet]

do your insulation tests first these should show any problems i don't understand why you are testing the voltages because these may not be actual true voltages and just induced votages

I would not do any insulation testing until all circuits have been identified by continuity first, but I do know what you mean :innocent

 

[a1spark]

I agree on the induced voltage theory in regards to the unusual voltage readings to the cps on the circuits described in the post.Insulation tests would be advised ,as mentioned above, and also cpc continuity testing back to the MET is a must

 

[Nicky Tesla]

You are picking up induced voltages on the earth. Check continuity

The fan may be U/S that is why it is blowing fuses. Check L/N resistance and check L/E for a short as a starter. If there is not a good earth then the leakage may be enough to blow a 13a fuse but not melt tin foil.

Is there an RCD?

 

[matt.leung]

it is RCD'd but the hood has no earth, unless it was cut right back within the cable.

 

[Sidewinder]

The old induced or phantom voltages again. One major issue with these new fangled digital multi meters/testers.

Hence why Steps is such a fan of somethign that pulls a little current from the circuit!

Neon indicators (NOT screwdrivers) Martindale, the old SqD rattler and others have the advantage of drawing a bit of current from the circuit, thus collapsing phantom or induced voltages to zero as there is no current there to sustain the voltage under load.

However, the designed high input impedance of digital meters does not allow this to happen.

Now this is good in certain situations, think Heisenberg.

However, it can cause confusion when undertaking certain tests.

One reason I have just bought an AVO9 MkII for fault finding!

O) ] :innocent

Paul

 

[Nicky Tesla]

You could get a 100k resistor attached to small lengths of wire with croc clips on the end. Attach one end to an earth and the other to the cpc and retest at the cpc side.

 

[green-hornet]

The old induced or phantom voltages again. One major issue with these new fangled digital multi meters/testers.Hence why Steps is such a fan of somethign that pulls a little current from the circuit!

Neon indicators (NOT screwdrivers) Martindale, the old SqD rattler and others have the advantage of drawing a bit of current from the circuit, thus collapsing phantom or induced voltages to zero as there is no current there to sustain the voltage under load.

However, the designed high input impedance of digital meters does not allow this to happen.

Now this is good in certain situations, think Heisenberg.

However, it can cause confusion when undertaking certain tests.

One reason I have just bought an AVO9 MkII for fault finding!

O) ] :innocent

Paul

Sometimes, just sometimes I get envious at your logic whilst people get rid of old testers some bright spark comes up with a perfect example of why you should keep them.

However under normal conditions I guess most people would be ok with multi's its only when anomalies come up that they can be a very good time saver.

(sharing this one with KME or is that just love you share?)

 

[PIP IOM]

make sure no appliances ie cooker etc are not reverse polarity connected. I know i'm stating the obvious but i came across it on a TT job. Cooker L+N reversed .

 

[Sidewinder]

I knew straight after I posted it I should not have put that comment up!

Course it was too late then!