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seajayess

Question

Ok last Saturday we had to install a new supply to a large freezer in a food processing factory, so 10mm 5 core SWA from a Sub board to control panel all tested out fine and left isolated for refrigeration engineers to finish of their installation.

Got called back today as LED's on panel were glowing although panel was still isolated. After 2 hours trying to find out why ended up with only 1 explanation, that due to the amount of currents in the factory there was some current traveling up the Neutral enough to light LED's.

But while testing the SWA with the cable completely removed from C/U and freezer panel so just a piece of SWA with nothing connected to it you could get 238 volts between it and the supply line conductors and nothing to Neutral or Earth so why is there a potential difference.

How is this possible when there is no connection to anything ?

To me it is somehow connected to Earth, but how. We did an insulation resistance test on it and there is no connection to anything in the building.

Hope this makes sense CJS

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Thanks Special Location you have answered my question that is exactly what I am getting just seams strange to me but that is whats happening give yourself a Guiness Drink

The LED's I think are getting an imbalance current through the neutral from the 3 phase supply as the line conductors are isolated and would only require milli amps to light and they take a while to eliminate CJS

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I know it's a factory but I'd have a tester across the supply on the cable and flick off each of the circuits on the board individually (where possible) to see if you can isolate where the voltage is coming from.

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Have had induced voltage causing LEDs to glow dimly before, but never with such a high voltage showing. Think elvis has the right idea.

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I am of the opinion that my learned friend; trailer boy, has it spotio onio. I couldn`t have vocalised it as well as he just did (double scoobies to you, my friend!), but was trying to get my head around a similar description.

KME

Oh come come now mr KME... we all know your scheme....X(

You thinks! Ah I know the answer to this... but if I spend ages typin it out...

I shall have less Beer drinking time!:_|Guiness Drink:(

So I shall wait until that gullible little four legged furry pig thing, types a load of stuff out then just either agreed or disagree as appropriate...

Let him wear his typin fingers out.. while I keep my beer drinking muscles up to scratch!

You have been routed Mr KME!!!!X(

ANYWAY! you promised my typing lessons a long long time ago in a far off galaxy!!!X(

gimmey my fast typin fingers!!!!!!:o:(

or else!X(

:coat

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3 Phase PME and at the mains there is on average 700 Amps per Phase but the cable was not connected to anything. And it cant be induced as the voltage is not on the cable as I said no voltage between cable and N or Earth it is like the cable is at 0 voltage CJS

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3 Phase PME and at the mains there is on average 700 Amps per Phase but the cable was not connected to anything. And it cant be induced as the voltage is not on the cable as I said no voltage between cable and N or Earth it is like the cable is at 0 voltage CJS

Daft question, but what electronics (if any) exist in the unit?

Don

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I know it's a factory but I'd have a tester across the supply on the cable and flick off each of the circuits on the board individually (where possible) to see if you can isolate where the voltage is coming from.

Excellent.

(Assuming, the site can be isolated without loss of production?)

The Godfather

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The cable was not connected to anything, used a volt meter one probe on any phase the other probe on the SWA that wasn't connected to anything so should be no potential difference even removed it from the glands so even the steel wire wasn't connected and as I said meter read 238 volts CJS

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Ok people you all seem to be thinking in the wrong direction. There isn't an induced voltage of 238 volts, if anything it is at 0 volts. The cable is removed from everything so just like a piece of SWA lying through the building and def not touching anything, and the cable is def not damaged in any way.

The voltage we are getting is from a live source to the SWA internal conductors, which is NOT connected to anything even the steel wire armouring. So why are we getting the potential difference of 238 volts CJS

Right... Let me see if I can get me head round what you are saying here.

Q1/

I couldn't see any reference as to how long this run of SWA is?

I assume its long!

Q2/

I couldn't see any detail about the "type" of meter you are using?

I assume a typical high impedance multimeter,

(e.g. one which would put approx 10megohm - 15megohm across the voltage probes, when taking volt readings)?

Q3/

I assume your reading is 238v 'AC' NOT 'DC'?

Q4/

Are you saying you have one meter probe on any one of the cores of the SWA

and the other probe on a "LIVE" (230v) conductor somewhere on a local supply?

OR is the second probe on a 'Neutral' or 'Earth' conductor?

I read it that you are across 'Live' (230v) & a core of the 'open circuit/disconnected' SWA ?

IF all of my assumptions are correct....

Then your 'apparent' 238v reading probably sounds right to me?:|

e.g.

You have one ruddy great big aerial cable lying across some part of the factory.

You are measuring the centre core of your aerial against a reference point that is fluctuating between, 0v, +230v, 0v, -230v, 0v, +230v, 0v, -230v etc.. etc...

Whatever potential your aerial is at..

it is being measured against a varying reference point ..

thus an apparent 230v signal.

Go try it at home on your analogue TV aerial...

Disconnect all you TV gear.

Put one meter probe on the centre pin of the "open circuit" aerial down feed termination.

Put other meter probe on a 230v live supply from a nearby socket outlet.

you will get an apparent 230+v between a "live source" and your Open circuit cable.. just lying there connected to nothing.

Thats all I can think of from info given?:|

Now...

If I have my pants on backwards and you are referencing your SWA against Earth or Neutral... you wont get the 230+v:(

Then I would need to go back in me box and think again...:_|

probably need a bit more info though!?:o?:|

:coat

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The cable was not connected to anything, used a volt meter one probe on any phase the other probe on the SWA that wasn't connected to anything so should be no potential difference even removed it from the glands so even the steel wire wasn't connected and as I said meter read 238 volts CJS

So if your wiring was tested OK, then the Refrigeration Engineers come in and you now have a a fault?

The Godfather

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Ok people you all seem to be thinking in the wrong direction. There isn't an induced voltage of 238 volts, if anything it is at 0 volts. The cable is removed from everything so just like a piece of SWA lying through the building and def not touching anything, and the cable is def not damaged in any way.

The voltage we are getting is from a live source to the SWA internal conductors, which is NOT connected to anything even the steel wire armouring. So why are we getting the potential difference of 238 volts CJS

Not ALL thinking in the wrong direction.........

ARE you saying you put one probe of the volt tester on a live terminal somewhere on the installation and the other probe onto the SWA armour?

If yes you would expect the mass of the SWA to act as a crude but poor earth rod and give you the potential difference you describe. No?

:|

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Is the cable running near other cables?

But isn't 238V rather high for any mutual conductance?

The Godfather

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Its wierd CJ , but inductance is the only answer. I presume it will not power a light bulb. Are there any non SWA cables in the same run ?

There is a post of mine about 70mm meter tails interfering with a PC monitor screen through a solid concrete block wall, it was because I had run them seperate on the tray instead of bunching them . After reclipping them tight together the inductance was reduced to a few inches.

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Are you saying it is effectively earthing through the air Apache ? I have seen when fault finding panels floating voltages, when circuits are not pulled down to earth.

A similiar effect ?

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I don't think there is a fault, I just wanted someone to tell me why this was happening , its like its getting an earth inductance , is something to do with the building be steel framed , the other sparky I was with has been a sparky for 25 years and he cant explain it, just thought somebody on here may be able to CJS.

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Are you saying it is effectively earthing through the air Apache ? I have seen when fault finding panels floating voltages, when circuits are not pulled down to earth.

A similiar effect ?

of just the general mass of the SWA?

There will be a decent mass of steel, CJS doesn't say what size or how long, but we would have a good few kilo's of mass that would only need to take a few miliamps to give a voltage reading.

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But isn't 238V rather high for any mutual conductance?

The Godfather

Sounds rather high but I had over 90v on a circuit the other day after connecting cpc to bar there was no volts but the wiring is a bit dodgy on the job.

Batty

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Thanks Special Location you have answered my question that is exactly what I am getting just seams strange to me but that is whats happening give yourself a Guiness Drink

The LED's I think are getting an imbalance current through the neutral from the 3 phase supply as the line conductors are isolated and would only require milli amps to light and they take a while to eliminate CJS

hmmmm have another Guiness Drink

can I refuse that offer.....

I shall sneak another one in while the Don is not looking thank you sir!Blushing:x

Guiness DrinkGuiness DrinkGuiness DrinkGuiness Drink:pBlushing

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Sounds rather high but I had over 90v on a circuit the other day after connecting cpc to bar there was no volts but the wiring is a bit dodgy on the job.

Batty

My Boys tell me they wouldn't expect much over 150V at worst and typically around 100V.

The Godfather

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