is this a faulty FCU?

[propertydev83]

I have a switched and neon fused connection unit on a ring main. When the switch is in the off position and checking the Load live and neutral connections with a voltmeter, both are giving a 240V reading. When the switch is in the on position the live gives 240V and the neutral gives 0V, as expected. Is the FCU faulty?

Background on this:

The FCU is for a cooker hood. When it was first switched on it blew the 3A fuse straight away and knocked the power out. It did this a couple more times after checking the wires going into the FCU and trying again.

Seemed it must either be a faulty hood or the flex connections in the chocbox on the spur from the FCU. The flex wires were removed and refitted, and after that the.fuses stopped blowing and the hood is working

Later when the lights were off I noticed the neon had a faint glow while the switch was in the off position. This is when I checked the connections with the voltmeter and got 240V on both live and neutral. Also noted that pressing any of the buttons for the fan or lights on the hood would not make the hood operate, and would still get 240V readings on both live and neutral, but it would cause the neon to stop glowing.

Removing the fuse would also cause the neon to stop glowing, and give 0V on both live and neutral. Whereas removing the Load wires that connect to the hood would also cause the neon to stop glowing, but would continue giving 240V on live and neutral.

I have another FCU on the ring main with nothing connected to Load yet, so I checked that one's Load connectors with the Voltmeter. I think this one was giving a low/>12V on both live and neutral when switched off, if I remember correctly... (which still seems wrong to me aswell.)

The only thing I haven't tried yet is taking the FCU off and putting a different one in its place.

 

[rjm2k]

A FSU costs less than 3 quid, worth a try even just to help fault finding.

 

[propertydev83]

I have a couple of spares for later use so I will try one of them when there's daylight again, but I thought no harm in asking, and was concerned of any "danger" posed by this fault.

 

[JohnW2]

I have a switched and neon fused connection unit on a ring main. When the switch is in the off position and checking the Load live and neutral connections with a voltmeter, both are giving a 240V reading. When the switch is in the on position the live gives 240V and the neutral gives 0V, as expected. Is the FCU faulty?

What sort of voltmeter? Assuming it's one with two leads/probes, where are you measuring these voltages - 'between what and what'?

Kind Regards, John

 

[propertydev83]

Sorry, no it's a "Digital Voltage Tester":

 

[PrenticeBoyofDerry]

You need proper test equipment, which will have two probes for testing, not that unsafe shyte you are using!

Untitled

• PrenticeBoyofDerry
• 22 Mar 2014
• 1

 

[JohnW2]

Sorry, no it's a "Digital Voltage Tester":

As I rather suspected You are probably simply being misled by the readings from a device which is inappropriate for this purpose. I obviously can't know for sure, but the chances are the there's nothing wrong with the FCU.

Kind Regards, John

 

[propertydev83]

I appreciate that.

But even so, as I understand it a switch should "isolate" part of an electrical circuit (Or switch to use a different part of it)? It is double pole so both live and neutral connections should also be broken.

I can't understand how the neonl can glow when this part (spur) of the circuit should be totally isolated, and how I get any reading anywhere there at all whatsoever (despite the tool being used)? It should be totally dead since the only thing connecting these wires to the circuit is this switch, which is off...

(My linited kbowledge tells me something inside the switch itself making a connection it shouldn't or an arc fault )

 

[flameport]

The likely cause is the the FCU is only switching the neutral and not the line.

The FCU needs to be replaced.

 

[JohnW2]

I appreciate that. But even so, as I understand it a switch should "isolate" part of an electrical circuit (Or switch to use a different part of it)? It is double pole so both live and neutral connections should also be broken.

All true.

I can't understand how the neonl can glow when this part (spur) of the circuit should be totally isolated, and how I get any reading anywhere there at all whatsoever (despite the tool being used)? It should be totally dead since the only thing connecting these wires to the circuit is this switch, which is off...

The load-side terminals may be isolated/'totally dead', yet it's in the nature of the sort of voltage detector you're using that you might still 'detect voltage'. I obviously cannot say that the switch in your FCU isn't faulty, but what I am saying is that it's far from impossible that you could get the results you've seen even if the FCU were functioning correctly.

In the absence of proper measuring equipment, your plan to at least temporarily replace the FCU tomorrow should go a fair way to answering the questions.

Kind Regards, John

 

[333rocky333]

As above, sounds like the fault current has welded the live contacts in the switch together.

As switched FCUs are double pole its switching neutral only, the reading your getting is from the live passing through the appliance back to the LOAD N terminal.

Is that tester that clever it reads 240 V using just one probe?

EDIT
missed this bit

Whereas removing the Load wires that connect to the hood would also cause the neon to stop glowing, but would continue giving 240V on live and neutral.

If thats correct then as John W says your being misled by that tester

 

[propertydev83]

The likely cause is the the FCU is only switching the neutral and not the line.

The FCU needs to be replaced.

From what I read in another thread about "explaining neutrals" this seems to make sense.

(Broken neutral when "off" prevents the current flowing or powering the hood on its way, but the connected line when "off" means there is still 240v in the line, hood, and the neutral, just unable to send a current anywhere.)

 

[JohnW2]

As above, sounds like the fault current has welded the live contacts in the switch together. As switched FCUs are double pole its switching neutral only, the reading your getting is from the live passing through the appliance back to the LOAD N terminal.

That's obviously all credible. The OPs 'replacement' test should resolve that. However, it sounds as if you and flameport have not seen the sort of seriously misleading results that I've seen some people get with such 'voltage detectors' - maybe the cases I've seen have been 'extreme'/unusual!

Is that tester that clever it reads 240 V using just one probe?

I'm not sure that 'clever' is necessarily the word I would use

Kind Regards, John

 

[333rocky333]

To the op
Thats correct but this would not happen

Whereas removing the Load wires that connect to the hood would also cause the neon to stop glowing, but would continue giving 240V on live and neutral.

 

[JohnW2]

From what I read in another thread about "explaining neutrals" this seems to make sense. ... (Broken neutral when "off" prevents the current flowing or powering the hood on its way, but the connected line when "off" means there is still 240v in the line, hood, and the neutral, just unable to send a current anywhere.)

That's all true - but, as 333rocky333 has said in his edit, if that were what were happening, you would not still measure 240V at the neutral load terminal when you removed the load cable.

If I were a betting man, my money would still be on your being misled by your 'tester'.

Kind Regards, John