Bell Transformer

[tonyelectric]

Problem with clock outlets is they cost a packet these days (£22 last time I purchased one) because they hardly see any use in the age of quartz clocks, but you can get BS1362 fuses in 1A for a spur if you feel so inclined and there is even a 2A plug with a fuse in it made (by Mk if memory serves me right)

 

[bernardgreen]

Connecting one side of it to earth won't help either, as the size of the SELV wiring would probably be too small to carry the fault current.

The fault current only needs to be 30 mA to trip the RCB and I am sure the SELV wiring will carry 30 mA

And double insulation on an SELV cable going to the transformer in the CU is not going to be effective for long if the SELV cable is touching a 230 volt conductor hot enough to melt and may be ignite its insulation due to a loose terminal.

And MCBs and RCBs are not tripped by a loose connection becoming over heated.

 

[bernardgreen]

This is especially daft as quite a few brands of MCB are the same product made under different badges

The products may have the same moulded case and made on the same production line but the internal design and specifications for materials used and quality control procedures may vary considerable depending on which brand label is being applied to the casing.

 

[flameport]

The fault current only needs to be 30 mA to trip the RCB and I am sure the SELV wiring will carry 30 mA

The wiring would of course carry 30mA, however if something at 230V contacts an earthed wire, the fault current will be many thousands of times greater than that.

 

[tonyelectric]

This is especially daft as quite a few brands of MCB are the same product made under different badges

The products may have the same moulded case and made on the same production line but the internal design and specifications for materials used and quality control procedures may vary considerable depending on which brand label is being applied to the casing.

Yes that is possible, however I suspect a Steeple breaker and a Newlec one would be identical inside, it would cost more to tool up for different batches, in any case they all have to comply with the BS & CE requirements for the catagory of product so safety should not be impared, it may simply have a shorter working life

 

[bernardgreen]

The wiring would of course carry 30mA, however if something at 230V contacts an earthed wire, the fault current will be many thousands of times greater than that.

And therefore trip the RCB removing all 230 power.

If the wire of an SELV circuit comes into contact with 230 volt then the entire SELV circuit is at 230 volts above earth potential and there is no way that this can be detected and the 230 volts removed automatically. In the case of a door bell button that is wet with rain that could result in electric shock to some one pressing the button.

SELV is specified as the circuit being "Safe" or "Separated" in the event of any single fault. And for most faults that is true. But is a melt out of insulation on a 230 volt conductor that also melts the insulation on an SELV conductor and results in the SELV conductor becoming live from the damaged 230 volt conductor a single fault ? Apparently it can be considered as two faults. Two separate wires have lost insulation at the same time so two faults and outside the single fault specification of the SELV.

 

[ericmark]

To earth, or not to earth: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous SELV,
Or to take connections against a sea of troubles,
And by opposing end them? To die: to sleep;
No more; and by a sleep to say we end
The heart-ache and the thousand un-natural shocks.

I remember as a kid I could not understand earthing as it seemed to my uneducated mind if we did not earth then we could not get a shock by touching earth. The point is of course we use more than one item and unless we earth then we end up with multi-faults which then causes the danger.

Using a shaver socket we do remove the earth and supply just one item per secondary winding and that is deemed safe.

However it is one method or the other and to earth one output of a shaver socket would remove the safety feature and the same although to a lesser extent is true with SELV.

The whole idea is if we touch either of the cables it will not pass any current as there is no return path. Although likely with the door bell there will be no problem with other items especially where used in a bathroom introducing an earthed wire means it can react with other items and cause a problem.

Years ago before double insulated drills were the norm I had many problems were welding had caused very high current to travel in the earth wire and burn out the earth. I am sure the drill manufacture when designing the drill never expected the drill to have that problem and it is of course the same with most accidents something happens which we had not allowed for.

Therefore I would never earth intentionally any SELV item.

 

[bernardgreen]

Earthing one side of the isolated 230 volt shaver output means the other side is 230 above earth and therefore potentially lethal if an earthed person touches that side.

But earthing one side of a 12 volt transformer output means the other side is 12 volts above earth and while an earthed person touching that side will receive a voltage and a current will pass through the person it will be a current that will be far to low to be lethal.

and it is of course the same with most accidents something happens which we had not allowed for.

or decided is so unlikley to happen there is not need to protect against it.

 

[tonyelectric]

The wiring would of course carry 30mA, however if something at 230V contacts an earthed wire, the fault current will be many thousands of times greater than that.

And therefore trip the RCB removing all 230 power.

If the wire of an SELV circuit comes into contact with 230 volt then the entire SELV circuit is at 230 volts above earth potential and there is no way that this can be detected and the 230 volts removed automatically. In the case of a door bell button that is wet with rain that could result in electric shock to some one pressing the button.

SELV is specified as the circuit being "Safe" or "Separated" in the event of any single fault. And for most faults that is true. But is a melt out of insulation on a 230 volt conductor that also melts the insulation on an SELV conductor and results in the SELV conductor becoming live from the damaged 230 volt conductor a single fault ? Apparently it can be considered as two faults. Two separate wires have lost insulation at the same time so two faults and outside the single fault specification of the SELV.

But surely this is why we run extra low voltage cables such as bell wire, telephone cables, impulse clock circuits in separate conduits and ducts or away from the mains (that and to avoid interference), I guess the ultimate then would be to sleeve the bell wire in the CU with glass fibre heat resist and or ensure they are dressed so as ot to physically make contact with the outer sheath of a 230V carrying wire.

 

[PrinceofDarkness]

Bernard: you're a danger to yourself and to your customers if you adopt this cockeyed logic to earthing. You're also in violation of the Regs and of Commonsense by doing so.

A typical 12V separated or 'Isolated' output can do no harm in the event of a fault. But if you're stupid enough to earth one side of a SELV output - particularly on a PME supply, then you're introducing a very real danger in the event of a lost supply neutral.

So, stop this stupid practice, before you kill someone!




Lucia.

 

[plugwash]

there is even a 2A plug with a fuse in it made (by Mk if memory serves me right)

MK list one but i'm not aware of anywhere selling it anymore (CPC used to)

 

[bernardgreen]

But if you're stupid enough to earth one side of a SELV output - particularly on a PME supply, then you're introducing a very real danger in the event of a lost supply neutral.

And in the event of a lost supply neutral on a PME what happens to those services that are bonded to the earth terminal ?

Not to mention any metal lamps that are earthed to the earth terminal.

So, stop this stupid practice, before you kill someone!

Noted

 

[PrinceofDarkness]

The theory is, bernard, that the (unlikely) loss of a PME supply neutral would result in the bonding conductors acting as the return path. That's why the Supply Regs are (or have been 'tll recently) quite strict on the size of bonding conductors relative to the supply neutral and the effectiveness of their connections.

I apologise for calling you 'stupid', it was quite rude of me ( I only do it when I've been on the 'Gordons'). But you must realise that a SELV circuit is specifically designed not to have any reference to Earth and to introduce an earth in that stuation would be futile because an ELV output fault couldn't possibly activate an RCD on the LV input side.


It isn't a very smart idea to tinker with tried and trusted methods of supply.



Lucia.

 

[ELZ4742]

So a bell transformer made by Friedland can't be put inside a MK consumer unit unless you have written permission to do so.

really? even though they are the same company? strange.

 

[bernardgreen]

It isn't a very smart idea to tinker with tried and trusted methods of supply.

But there is no harm is asking questions as to why they are considered to be trust worthy.

The idea and aim of having all inside the house at equi-potential and conecte to the imported earth is fine for people inside the house. In the event of a fault where the neutral is lost then everyone inside the house is safe even if the imported "earth" is connected to a phase. But cold water tap on the outside of the house is now potentially fatal. So isn't there some value in thinking about that possibility and making some arrangements to reduce that hazard. Like wise there is some value in considering the possibiity of an SELV circuit comming in contact with a phase.

But you must realise that a SELV circuit is specifically designed not to have any reference to Earth and to introduce an earth in that stuation would be futile because an ELV output fault couldn't possibly activate an RCD on the LV input side.

A fault inside the ELV circuit could not trip the RCB. But a fault of contact between the ELV and a phase will raise part of the ELV circuit to phase voltage and other parts to phase voltage plus or minus the ELV voltage. The voltage on the ELV is now phase and referenced to earth and a potentiallly fatal circuit is possible. The RCB might trip if any current flows from phase to earth via the ELV circuit and a human body. If the ELV circuit is earthed intentionally when installed then the RCB WILL trip when the phase comes into contact with the ELV circuit. ( provided the ELV circuit can carry the 30mA required to trip the RCB. )