Consumer Unit Main Switch

[jcapes]

Hi everyone. Im having a bit of a problem understanding the function of the main switch in a consumer unit and would like a detailed explanation of why its there.

I understand that they can incorporate RCD protection, however does a standard main switch offer any protection at all (overload or fault) or is it simply a isolation device with a raiting on it to show how much current it can operate with. They have a 100a raiting which makes me think if the installation was overloaded as a whole (over 100a) without overloading any particular circuit would the main switch trip like a standard MCB protecting an individual circuit out or would it just bypass it (main switch staying closed) and in turn trip out the suppliers fuse? I understand that its not very likely for a domestic property to overload like this as not all circuits are used to their max at the same time but for me to understand this properlly it would be good to get a bit of a clear explanation from someone who knows. I hope im explaining myself clearly and i hope you can help me. Thank for reading

 

[Taylortwocities]

It is just a double pole switch. If it says 100A then that is just the rating of the contacts.
There is no over current or earth leakage features, its a switch, it isolates the supply, that's it.

 

[EFLImpudence]

I understand that they can incorporate RCD protection,

Some installations may, in the past, have used a RCD as a main switch (without the type you are asking about) but this is no longer considered acceptable for safety reasons as one fault will disconnect every circuit in the property.

 

[jcapes]

Thanks for your replies to this. I now understand that a basic mains switch offers no protection it's simply the rating of the contacts of the switch. Could you answer what would happen if the installation was overloaded? Would it blow the suppliers fuse ? Thanks for all your help.

 

[ban-all-sheds]

Why do you want to know these things?

 

[Taylortwocities]

Lets not over-complicate this.

If the device says BSEN 60947-3 then its a plain and simple switch.

Let us know if it has a different BSEN number.

 

[Taylortwocities]

If the installation was overloaded then the next protective device will be what will blow.

In the case of a domestic premises it will be the supplier's service fuse.

But yes, why do you need to know?

 

[jcapes]

I don't really need to know, however I recognise small gaps in my understanding which are usually very simple and small but annoy me !! That's all. Thanks for all your help.

 

[ban-all-sheds]

Why am I worried that someone who doesn't know what a main switch is, or does, or why it's there or what can happen if an installation is overloaded feels the need to ask about those things to fill in what he thinks are small gaps in his understanding?

 

[JohnW2]

Why am I worried that someone who doesn't know what a main switch is, or does, or why it's there or what can happen if an installation is overloaded feels the need to ask about those things to fill in what he thinks are small gaps in his understanding?

With respect, I think you're being very unfair to him. Eveything he said in his initial post was perfectly reasonable, and indicated a reasonable degree of understanding. The only thing he didn't know, and said he didn't know, was whether a main switch was simply an isolating device or whether it offered any sort of protection (overload or fault). He even used the right vocabularly/jargon ('isolation device', 'protection', 'overload or fault'), which ought to keep many of the folk here happy.

As for his question .... the visible part of a main switch looks much the same as a double-width MCB, so I don't think it unreasonable for someone who doesn't know to wonder whether it might be, say, a double-pole protective device, as well as providing a manual means of isolating the whole installation. [In fact, on reflection, I'm not so sure that having a DP MCB as a main switch would be all that bad an idea!].

A desire to increase one's knowledge should be encouraged and assisted, not 'worried about'!

Kind Regards, John

 

[jcapes]

Hi John, thanks for your comments. I'm not a practising electrician at the moment. I'm just interested in possibly taking this career path in the future and because of that done a few courses etc to improve my knowledge that's all. Thanks for your understanding and help

 

[JohnW2]

Hi John, thanks for your comments. I'm not a practising electrician at the moment. I'm just interested in possibly taking this career path in the future and because of that done a few courses etc to improve my knowledge that's all. Thanks for your understanding and help

You're very welcome. I'm not a professional electrician either, and never will be. Most people here are pretty well-meaning, and many are very experienced and/or knowledgable - but initial responses to questions can sometimes appear inappropriate (for a forum specifically created for non-electricians), intimidating or sometimes even aggressive, which is a great pity. However, if you have a hard enough skin to ignore those 'rough edges', it is a very valuable resource.

Kind Regards, John

 

[JohnW2]

Well John what courses do not metion isolation?

I'm a bit lost - what have courses got to do with this? This was simply a question to satisfy a desire for knowledge from 'Joe Public'.

In any event, the op did implicitly acknowledge that the main switch served, at least, as "simply an isolation device" (which presumably means he knows what that means, and does) but was wondering whether t also afforded any (overload or fault) protection.

Kind Regards, John.

 

[ajrobb]

My understanding is that mains isolators incorporating 100 mA RCD are used with TT supplies and a local earth as the local earth cannot take the full fault current. They are only needed if there are some circuits not protected by 30 mA RCD/RCBO.

 

[1john]

My understanding is that mains isolators incorporating 100 mA RCD are used with TT supplies and a local earth as the local earth cannot take the full fault current. They are only needed if there are some circuits not protected by 30 mA RCD/RCBO.

That still wouldn't comply.

100mA RCD is used to meet disconnection times, why wont the local earth take the current?

RCD's are used on a TT system due to the normally high external earth fault loop impedance readings which result in a fault to earth not clearing the protective device in time.

If a circuit is protected by a 30mA RCBO only it MUST be double poles, the majority available now are not.