Obvious question more than likely.

[jwc]

Second question of mine on here and it's one I've never got round to getting an answer for.


Mains comes into a building through a 60A main fuse for instance.
Why then are main switches rated at 100A or 80A?
Does this mean any load above 60A would blow the main fuse before it blew the main breaker?


Don't be harsh people. I'm fully aware the answer's probably an obvious one.

 

[telectrix]

you are confusing the main switch with a fuse. the rating of the switch is the max. load it will carry. it is not an overcurrent protective device

 

[xylophone]

Hi. The answer is in the terminology. When you write "Why then are main fuses rated at 100A or 80A?" you probably mean "Why then are main switches/RCDs rated at 100A or 80A?"
The switch rating is not related to your supply rating (but needs to be same or more)

 

[jwc]

But why have a main switch rated at 80A if the main fuse is only 60A?

 

[telectrix]

60A fuse will happily run @ 80A for about 1 hour. that's why you have a switch rated above the fuse rating.

 

[jwc]

Just received an inbox stating that the main switch doesn't trip on overload? :/
Really? Thought the main switch was essentially an MCB to carry the load of the other MCB's if that makes sense?

 

[dan86]

at the moment you're making no sense man

 

[jwc]

How? haha.
I just wanna know what's the point in having a main switch with a higher rating than the main fuse which supplies it?
Surely you want your main switch to disconnect before the main fuse blows?

If your main fuse is 60A, main switch is 100A, and your house is pulling 80A, what use is the main switch?

 

[telectrix]

Just received an inbox stating that the main switch doesn't trip on overload? :/
Really? Thought the main switch was essentially an MCB to carry the load of the other MCB's if that makes sense?

totally wrong. the switch is a 2 pole switch, nothing more. each circuit is protected by it's own MCB. the installation as a whole is protected by the DNO fuse. think of the switch as your car. it will do 110 mph, but you run it at no more than 70mph.

 

[jwc]

Thanks for explaining, Telectrix.

 

[Darkwood]

JWC - Its hard to respond to a question sometimes without been harsh but im scared to the fact this question comes up numerous times from 'Qualified' Electricians now im starting to think you in yourself are not to blame here but either your tutor or the course itself .... may i ask what kind of course you have taken... now you realise that the main switch is just a mechanical switch and nothing more its brings to question are other areas of your learning lacking in basics ..... this could be dangerous in design in some situations if you thought the main switch on a standard dist-board afforded overload protection.

Hope you dont take as a dig at you...

To note hear you can get main switches although usually in larger commercial and industrial boards that do afford overload protection 'MCCB's and or a combination of earth leakage protection.... maybe this is where you may have confused yourself, but your bog standard 17th edition board DP switch is exactly that - a switch without the perks.

 

[jwc]

Thanks for the explanation, Darkwood. I've completed a full apprenticeship, AM2 and NVQ Level 3. Everything's much clearer now knowing that the main switch is exactly that... A switch.

 

[shagbite]

Thanks for the explanation, Darkwood.

I've completed a full apprenticeship, AM2 and NVQ Level 3.

Everything's much clearer now knowing that the main switch is exactly that... A switch.

rubbish.

 

[trev]

Also not having a go but- How on earth can you think something called a main SWITCH is anything other than what it claims to be?

 

[jamesBoater]

How could such knowledge be missed or not taught if you have completed all the above, and gained the qualifications!

 

[ian.settle1]

On a similar vane I had an electrician with 2391 who carried out a PIR and recommended that a 40A 30mA RCD should be changed to a 32A one, asked him why he replied because the fuse protecting the circuit is only 32A thus it would go before the RCD if the was an earth fault and not the RCD as it was rate at 40A, shook my head and walked away in disbelieve

 

[Guitarist]

There is a simple reason for switches being 100A.... They can be used on a system with a 60A fuse, an 80A fuse, or a 100A fuse. In other words, one size fits all.
As already said, a switch is just that, the same way as an RCD is just that...they do not "trip" on overcurrent. While I agree that nobody knows everything, this is pretty basic stuff.

 

[SirKit Breaker]

But why have a main switch rated at 80A if the main fuse is only 60A?

Because thats what the manufacturer makes, and it will do for any supply rated at 80a or less.

 

[dan86]

I was refraining from saying anything, but it seems like the rest of you haven't held back! I agree with jamesBoater. how can the OP not know this basic knowledge after having gained said quals?!

 

[richy3333]

Thanks for the explanation, Darkwood. I've completed a full apprenticeship, AM2 and NVQ Level 3. Everything's much clearer now knowing that the main switch is exactly that... A switch.

FFS and people on this forum lambast Electrical Trainee!

Stick a 63A Main switch in and everyones happy?!