Fuses blow when I put phase and ground wires in contact even when light switch is turned off

[daz01FR]

Hello to all, I stumbled upon a strange problem this morning. I wanted to change the light bulb socket in my bathroom, when I noticed that when the ground and phase wires come in contact,the fuse-box trips even though the light switch is turned off. So I took a pen voltage tester and tested all three wires. When the switch is off, the pen doesn't light up on any of the wires. When the switch is turned on, the pen lights up only on the red phase wire. Everything looks normal at this point so what could be causing this?

 

[telectrix]

first thing is to relegate that tester driver to stirring tea dangerous and unreliable.

seriously... might be that the switch is in the neutral line.

 

[Strima]

Is it a circuit breaker tripping or an RCD?

For your own safety I would advise that to turn the supply of at the circuit breaker before working on your light fitting and test that the circuit is dead using a two pole voltage detector.

If you follow these steps this should ensure you're working safely:

1. Identify correct isolation point or device.
   For all work on low voltage electrical equipment or circuits, it is important to ensure that the correct point of isolation is identified. When isolating the main source of energy, it is also essential to isolate any secondary source (such as standby generators, uninterruptible power supplies and micro generators).
2. Check condition of voltage indicating device —such as a test lamp or two-pole voltage detector.
3. Switch off installation/circuit to be isolated.
   It should never be assumed that equipment is dead because a particular isolation device has been placed in the OFF position.
4. Verify with voltage indicating device that no voltage is present.
   It is important to ensure that the correct point of isolation is identified before proving dead. Adequate precautions should be taken to prevent electrical equipment which has been made dead, is carried out on or near that equipment, from becoming electrically charged during that work.
5. Re-confirm that voltage indicating device functions correctly on proving unit.
   Use proving unit to confirm that the voltage on the indicating device is functioning correctly.
6. In Germany, Switzerland and Austria, standards require additional steps, including:
   1. Carry out earthing and short circuiting.
   2. Provide protection against adjacent live parts.
7. 
   
8. Lock-off device used to isolate installation circuit.
   It is preferable for an appropriate locking-off device be used on the point of isolation.
9. Post warning notices.
   Suitable labelling of the disconnected conductors using a caution notice is vital to prevent the supply being reinstated.

 

[Wilko]

As per Tel, those things can be deadly. Definitely not to be trusted to confirm something is off / safe to touch. And they’re not much good as a screwdriver either , sorry .

Can you confirm from your fuse board exactly what has tripped?

 

[daz01FR]

As per Tel, those things can be deadly. Definitely not to be trusted to confirm something is off / safe to touch. And they’re not much good as a screwdriver either , sorry .

Can you confirm from your fuse board exactly what has tripped?

It's the RCD.

 

[pc1966]

Really you can't trust neon screwdrivers to prove anything is safe!

This is the sort of thing you should be using (trying, not very well, to find a French source):

Testeur de tension BE17

Testeur de tension CA/CC 12 - 400 V à LED avec indication de la polarité et fonction de test FI/RCD permettant de contrôler ou de déclencher les dispositifs DDR.

www.trotec24.fr

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Are there more than one light in that circuit?

Shorting phase or neutral to earth can trip RCDs, and that might be via live to another bulb to neutral.
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Just to add to what Strima as already made clear, you should prove your indicator is working before and after to "prove" something is dead. You can buy battery-driven proving units, but the cheap and nasty option it to check an unrelated socket is live to verify the voltage tester actually works.

 

[daz01FR]

Really you can't trust neon screwdrivers to prove anything is safe!

This is the sort of thing you should be using (trying, not very well, to find a French source):

Testeur de tension BE17

Testeur de tension CA/CC 12 - 400 V à LED avec indication de la polarité et fonction de test FI/RCD permettant de contrôler ou de déclencher les dispositifs DDR.

www.trotec24.fr

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Are there more than one light in that circuit?

Shorting phase or neutral to earth can trip RCDs, and that might be via live to another bulb to neutral.

I know I should be posting this on a french forum but I don't like french forums (who can blame me?). I was just wondering, assuming the tester isn't faulty, what could possibly be causing this.

 

[Lucien Nunes]

Shorting phase or neutral to earth can trip RCDs, and that might be via live to another bulb to neutral.

This is the specific answer to the original question. It suggests that your MCB is single-pole (some European installations use SPSN or DP MCBs) and only disconnects the line. When the short occurs, there is a path through any other loads for a small current driven by a small difference in voltage between neutral and earth, tripping the RCD. That voltage is too low to be detected with the handheld tester.

What was confusing was your comment that 'fuses blow when...' Normally we wouldn't use that description for an RCD trip because the MCB is equivalent to a fuse, not the RCD, and any short that trips an MCB tends to create a bang!

 

[daz01FR]

there's a second light in the bathroom. It's a neon light above the sink that's never worked. I don't know if it's on the same circuit as the main light but it has it's own little switch on the side. Could it be the culprit?

 

[Lucien Nunes]

There's no 'culprit', the behaviour is quite normal. It might be the power factor correction capacitor in it that is providing the path from switched line to neutral.

The only thing wrong in the present equation is using that nasty pen thingy to prove dead.

 

[daz01FR]

This is the specific answer to the original question. It suggests that your MCB is single-pole (some European installations use SPSN or DP MCBs) and only disconnects the line. When the short occurs, there is a path through any other loads for a small current driven by a small difference in voltage between neutral and earth, tripping the RCD. That voltage is too low to be detected with the handheld tester.

What was confusing was your comment that 'fuses blow when...' Normally we wouldn't use that description for an RCD trip because the MCB is equivalent to a fuse, not the RCD, and any short that trips an MCB tends to create a bang!

Sorry for the confusion.
Does the RCD trip because it detects a slight current going to the ground? because it says 63 Amps on it.

 

[Lucien Nunes]

RCDs used in the UK for additional protection trip at a level between 0.015 and 0.03A of current imbalance. Yours may be different according to the purpose for which it has been installed, although the trip current will be clearly marked on it as IΔn (I-Delta-n, the differential current). The 63A refers to the maximum load it is rated to handle and has no relationship with the tripping sensitivity

 

[daz01FR]

There's no 'culprit', the behaviour is quite normal. It might be the power factor correction capacitor in it that is providing the path from switched line to neutral.

The only thing wrong in the present equation is using that nasty pen thingy to prove dead.

If a capacitor discharges through me when i change a bulb in the bathroom I assume it could be dangerous. Is this normal? Because I really always just turn the switch off before replacing a bulb. Have I been playing with my life all that time? It says 63 Amps on the RCD.

 

[Lucien Nunes]

Capacitors in light fittings store very little energy and are normally designed self-discharge within seconds, even if they happen to be charged at the moment you open the switch. There might not even be one, especially if it is a small light. I was merely giving an example of a possible path for the trip current based on the little information I had about your bathroom!

 

[daz01FR]

RCDs used in the UK for additional protection trip at a level between 0.015 and 0.03A of current imbalance. Yours may be different according to the purpose for which it has been installed, although the trip current will be clearly marked on it as IΔn (I-Delta-n, the differential current). The 63A refers to the maximum load it is rated to handle and has no relationship with the tripping sensitivity

Okay I see it also says 30 mA in smaller type. I get that there's no danger then. I'm going to test with my tongue.
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Thank you all for the helpful replies. I guess I was just scared for nothing.

 

[Strima]

I'm going to test with my tongue.

I hope this is a joke.

 

[telectrix]

I hope this is a joke.

course it is. he meant a wet finger.

 

[Strima]

course it is. he meant a wet finger.

That's OK then.

 

[daz01FR]

I hope this is a joke.

It was, don't worry.

 

[Strima]

It was, don't worry.

Had to ask, someone out there would try it.