Electric shower switch repeatedly burning out. What can I do? :-(

[Harry Bloomfield]

I don't believe that's quite correct.

An isolator is designed to safely break the load, but not to make it.

No, they are only designed to carry the load, not make or break it whilst the load is enabled.

 

[Harry Bloomfield]

An isolator doesn't have the speed required to make or break a connection under load.

 

[Harry Bloomfield]

Some of the bigger high current switch/isolators are spring wound, push a button to operate quickly. In part it is the speed of the make or break which is the difference between a switch and a switch/isolator.

A slow make or break involves a lot of arcing.

 

[JohnD]

I remember being told that some isolaters will break a big load... but only once.

 

[DetlefSchmitz]

I don't believe that's quite correct.

An isolator is designed to safely break the load, but not to make it.

Not according to BS7671. Maybe you have different regulations where you do most of your work?

 

[JohnW2]

No, they are only designed to carry the load, not make or break it whilst the load is enabled.

Whilst that might be an understandable state of affairs (although not all that 'safe' since, in an emergency, people are quite likely to use a nearby isolator as a means of 'emergency switching'), as has been implied, it does not seem to be how BS7671 sees the situation. Virtually all of the devices listed in Table 537.4 as being suitable as a means of isolation are also indicated to be suitable for 'emergency switching' (which implies the ability to break a circuit under load) - which, in turn, implies that the relevant product Standards (as specified in the Table) require the devices to be capable of breaking cir circuits under load.

The only exceptions (devices OK for isolation but not for emergency switching) in the Table are the fairly obvious ones - primarily fuses and plugs/sockets.

Kind Regards, John

 

[DetlefSchmitz]

John, BS7671 says:
Disconnector. A mechanical switching device which, in the open position, complies with the requirements
specified for the isolating function.
NOTE 1: A disconnector is otherwise known as an isolator.
NOTE 2: A disconnector is capable of opening and closing a circuit when either a negligible current is broken or made,
or when no significant change in the voltage across the terminals of each pole of the disconnector occurs. It
is also capable of carrying currents under normal circuit conditions and carrying for a specified time current
under abnormal conditions such as those of short-circuit.

 

[JohnW2]

John, BS7671 says:
Disconnector. A mechanical switching device which, in the open position, complies with the requirements
specified for the isolating function.
NOTE 1: A disconnector is otherwise known as an isolator.
NOTE 2: A disconnector is capable of opening and closing a circuit when either a negligible current is broken or made,
or when no significant change in the voltage across the terminals of each pole of the disconnector occurs. It
is also capable of carrying currents under normal circuit conditions and carrying for a specified time current
under abnormal conditions such as those of short-circuit.

That is, indeed, how BS7671 defines a 'disconnector'/'isolator'.

I'm trying to work out the significance/meaning of "...or when no significant change in the voltage across the terminals of each pole of the disconnector occurs". That voltage (pd), will presumably be negligible when the switch is closed, and also (other than for induced/capacitively coupled voltages) when open (IF it is a DP switch) - but it is the bit between closed and open which we are potentially concerned about, isn't it?

In any event, I don't think this alters anything about what I recently wrote. As I said, with the exception of fuses and plugs/sockets, Table 537.4 says that any device suitable for isolation (hence, presumably, qualifying as an isolator/disconnector) is also suitable for 'emergency switching' (i.e. breaking the circuit under at least 'full load'), doesn't it?

Kind Regards, John

 

[bernardgreen]

in an emergency, people are quite likely to use a nearby isolator as a means of 'emergency switching'),

In a properly designed system the Emergency Stop disconnect should be by means of a Forced Break Contactor. This type of contactor forces the contacts apart by forcing an insulator between the contacts to prevent arcing and to break any weld that may have happened previously .

Not really economical for domestic installations.

 

[DetlefSchmitz]

What Table 537.4 is saying is that if you pick a switch to a particular other BS EN standard, then it may provide the functions of both isolation and emergency switching. What it does not say is that an isolation switch is automatically also an emergency switch, which is what was being partially claimed. The part about voltage simply means that you may use the switch in the additional case that the source voltage is already off (maybe by MCB), or not being drawn from (sinking device is already switched off itself).

 

[EFLImpudence]

What are you all arguing about?

Crabtree, or rather the sellers, describe their product as 50A 1-way pull cord switch and it is described as compliant with BS7671 for use as an isolator.
Using it as an emergency switch is different how exactly?

Selling a switch that should not be used for switching is ridiculous.

 

[DetlefSchmitz]

I have no idea what spec the Crabtree switch meets (may very well be designed to interrupt 50A). But BS7671 states that an isolator by definition is not expected to break any significant load current (as quoted above). I was merely taking exception to the statement that an isolator had to be able to break full load current and was therefore suitable as an emergency switch.

 

[JohnW2]

What Table 537.4 is saying is that if you pick a switch to a particular other BS EN standard, then it may provide the functions of both isolation and emergency switching.

It is.

What it does not say is that an isolation switch is automatically also an emergency switch, which is what was being partially claimed.

Agreed - it does not literally state that. However, unless there are some relevant Standards for isolators (or 'isolation switches' or 'disconnectors' or whatever) other than those mentioned in that Table, then it is effectively saying that anything which satisfies the requirements for an isolator is also acceptable as an emergency switch, isn't it?

The part about voltage simply means that you may use the switch in the additional case that the source voltage is already off (maybe by MCB), or not being drawn from (sinking device is already switched off itself).

That's certainly of situation which would satisfy that requirement. However, as I said, as far as I can make out, it would also be satisfied by a DP switch which precisely simultaneously broke both poles, since there would never be more than a negligible pd across the contacts of either pole, wouldn't it?

Kind Regards, John

 

[JohnW2]

In a properly designed system the Emergency Stop disconnect should be by means of a Forced Break Contactor. This type of contactor forces the contacts apart by forcing an insulator between the contacts to prevent arcing and to break any weld that may have happened previously . Not really economical for domestic installations.

Exactly, which is why the nearest 'switching device' (which will not correspond to what you describe) is what will be used in emergency situations in a domestic environment.

No-one can guarantee that a particular 'switching device' in domestic premises will not ever be used to break a 'full load', so it would seem inappropriate to allow any switching device to be installed in a domestic installation which could not be safely used for that purpose.

Kind Regards, John

 

[JohnW2]

What are you all arguing about?

I'm certainly not 'arguing' - I'm merely reporting what BS7671 says.

As I've just written, I would hope that we could all agree that it would be daft to allow any switching device in a domestic installation (and quite possibly others) that could not breaks the circuit's design current.

Kind Regards, John