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Topic Title: Voltage increase due to bad joint?
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Created On: 05 August 2014 10:18 AM
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 05 August 2014 10:18 AM
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sgtan27

Posts: 2
Joined: 25 July 2008

Hi,

I encountered a problem in a project involving LED Road Lighting.

The LED lights in a single feed flickered after installation. Investigation at site shows that the voltage at the AC termination of the luminaire to have a fluctuation upto 330Vac L-N, this is identified as cause of the flickering.

The lights when taken off and connected to a clean supply works normally.

The voltage reading at the Distribution Substation is a stable 240Vac L-N feeding the feeder pillar (FP)

The only problem seem to be a bad jointing on the feeding cable between the FP and the Lights.

How does a bad joint cause an overvoltage as seen on the load?
I am really concern that there is something else I am not seeing.

The LED lightings are connected individually to the alternate of the 3 phase in series using a 4C Cu cable. All sharing a common neutral.

There are about 21 lights in the circuit (7 per phase)

The earthing is a T-T with earthing provided at FP

Thanks
 05 August 2014 11:04 AM
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AJJewsbury

Posts: 11786
Joined: 13 August 2003

A bad joint on L wouldn't cause a raised voltage, but on an unbalanced polyphase system a dodgy N certainly can.

In the simplest case of a 2-phase system, loads are effectively connected in series across the full line-to-line voltage (e.g. 400V) - if the loads happen to be equal then both see half line voltage (e.g. 200V), but if you have a big load (low resistance) on one line and a small load (high resistance) on the other then the small load sees a bigger proportion of the 400V across it.

In a 3-phase system it's slightly more complicated with three lots of loads instead of two each trying to drag the floating portion of the N towards their respective live voltages and phase, so you're looking at vector addition rather than a simple voltage divider, but the principle and effect is the same.

- Andy.
 05 August 2014 09:29 PM
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slittle

Posts: 3598
Joined: 22 November 2007

Try measuring the Neutral to earth (a rod in the ground) and you will probably see quite a significant voltage on it.

As Andy has said a dodgy Neutral will result in the "neutral" on the load of the dodgy joint being pulled away from 0v by the connected loads.

Just had it on a site and had one phase going "down" whilst the other went "up" measured L-N. Measured L-E both phases went down a little bit on load

Stu
 05 August 2014 11:34 PM
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John Peckham

Posts: 7629
Joined: 23 April 2005

sqtan27

Welcome to the forum.

This 330V AC how was it measured? Is that an RMS voltage or a peak voltage measured with a power quality analyser?


The max. voltage from the DNO should not exceed 253V RMS. However that is RMS, the peak voltage is 253 x 1.414 = 357.7V.

If your 330V is peak then the RMS voltage is 233.4V which is a bog standard voltage L-N.

A loose joint in a neutral on a 3 phase circuit will cause over voltage problems. Also a loose joint on a supply to an inductive circuit may arc which could generate quite high short duration transients.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 06 August 2014 08:12 AM
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Jaymack

Posts: 4799
Joined: 07 April 2004

Originally posted by: sgtan27
The LED lights in a single feed flickered after installation. Investigation at site shows that the voltage at the AC termination of the luminaire to have a fluctuation upto 330Vac L-N, this is identified as cause of the flickering.

This is probably a 3Ph & N cable supply with tap offs at columns. The actual cause of the "flickering", is probably due to a faulty Neutral connection thus causing unbalance. It is a surprisingly common fault which should be recognisable immediately, sometimes the Neutral is not connected at all for a new installation.

Regards
 07 August 2014 11:48 AM
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sgtan27

Posts: 2
Joined: 25 July 2008

Yes, this is a 3Ph + N with tap off from the Ph - N at each column.
Basically 7 lamps in parallel on each Phase.

Rewiring the bad joint seem to have fixed the problem.
So the polyphase floating neutral is the explanation.

The 330V is RMS taken with a FLUKE multimeter.

Embarassingly a little more effort on google after reading your comments gave me an article that predicted this 330Vac L-N condition. and seem to suggest it as part of the detection of a floating neutral.

Thanks for the response.
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