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

[JohnW2]

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.

IF you are aware of any Standard for isolators that is not included in Tbale 537.4 of BS7671, then you might well be right to take exception to the statement - but, for what it's worth, I, for one, am unaware of any such Standard (which proves very little!!).

Kind Regards, John

 

[Harry Bloomfield]

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.

In an emergency, it doesn't really matter if the isolator is destroyed in the process. I've see lots of high current/high voltage switches, where an insulator is presented between the contacts as an arc breaker. I seem to remember having heard of, but not seen breakers where air is blown through the contacts, to disrupt the arc.

 

[DetlefSchmitz]

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?

I agree that that is the de facto situation if you take the contents of this 'for guidance' table as definitively representing the only standards allowed to be met.

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?

But you are ignoring the closing situation, which was also specified as a requirement. In this case there would most certainly be a voltage difference

 

[bernardgreen]

breakers where air is blown through the contacts, to disrupt the arc.

Often used on the open frame 132 kV and similar switches in open air substations.

Magnetic fields can be use to bend the arc into an arc which makes it longer and more likely to extinguish. For AC the magnetic field is often generated by coils of a few turns carryiong the current to the contacts, For DC currents metal magnets can be used,

One experience early in my career was to diagnose why the tool carrier on an automatic copying lathe had been driven into the rotating chuck. The operator had hit the Emergncy stop but the motors on the lathe continued to rotate erratically. It was an ancient machine used in WWII and then by a company that had bought the factory at the end of the war. The emergency stop contactor, an open frame device in the control cabinet, had been fitted with arc breaking coils but these had been removed as un-necessary when the contact pads had been replaced by larger ones. The error was thinking that the thick copper spirals were to conduct and disipate heat from the contact pads. The engineer was not aware they were magnetic items.

When the emergency switch was pushed the contactor dropped out but the arc persisted and carried enough current to keep the motors rotating until the damage was done.

That contactor opened by gravity and was closed manually and then held closed by an electromagnet.

 

[JohnW2]

I agree that that is the de facto situation if you take the contents of this 'for guidance' table as definitively representing the only standards allowed to be met.

Indeed - as I said, IF you can find some other Standard (not in that Table of 'guidance') which an isolator is allowed to conform to, but which does not require the device to be able to 'break on load', then the situation would be different.

But you are ignoring the closing situation, which was also specified as a requirement. In this case there would most certainly be a voltage difference

Am I? I realise that it is an argument of little more than academic interest but, as I said, there would obviously be minimal voltage across contacts when the switch was closed, and nor would there be any voltage across them if DP contacts opened exactly simultaneously (i.e. such that both load-side contacts were 'floating') - so I'm not sure what situation you are thinking of.

Kind Regards, John

 

[bernardgreen]

i.e. such that both load-side contacts were 'floating

The load side contacts would not be floating. They would have voltage until the two arcs were broken.

 

[DetlefSchmitz]

Thanks Bernard. I started to try to explain this - but then lost all enthusiasm.

 

[bernardgreen]

Thanks Bernard. I started to try to explain this - but then lost all enthusiasm.

I know the feeling. But today I am almost trapped at this computor.

I am in a conversation on a mailing list which is demanding rapid responses, so to fill in the time between questions I am on DIYnot for a change in outlook. Though I might just abandon DIYnot for a while and risk going to the kitchen and putting the kettle on.

 

[JohnW2]

The load side contacts would not be floating. They would have voltage until the two arcs were broken.

As I said, this is a side issue which is of no more than academic interest (in relation to the wording of the BS7671 definition), even if that. However, so long as the arcs exists, is that not effectively the same as the 'contacts closed' situation - i.e. with negligible pd between the contacts?

Kind Regards, John