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D Skelton

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So a conversation with a fellow spark brought into question what exactly are main switches?

Is a 100A main switch a 100A double pole MCB or does the 100A mean its breaking capacity? I'm sure it's the former but I'm beginning to question my own knowledge as he thought differently.

Also, with different rated RCDs, 25, 40, 63...etc, Does that mean that it will trip if the current passing through it exceeds that amount essentially acting as an RCBO? If so, why do we need RCDs to double up as overcurrent protective devices as well?

This is probably quite a basic question but the reason we wondered is because we all seem to fit CU's with 100A main switches even when the cut out fuse is only rated at 60A or 80A. If the main switch is indeed a 100A overcurrent protective device, why is this?
 
the 100A rating of the main switch is the rated current it can carry as a constant load. nothing to do with breaking capacity, as it's just a switch, nothing more. as for RCDs, the A rating is the max. safe load current.
 
100 amps is a fair load by any estimate. Regardless of what the mainswitch, mcb, mccb, or rcd's are rated at, its always good practice to isolate loads before you energise. Then ramp up the loads/circuits one by one.
I have seen the less experienced looking for a fault due to mccb's not latching back in. When in fact no fault existed, just a simple case of trying to energise under load.
 
To note here! A switch disconnector will have an AC rating and its this that denotes its switching under load capabilities -
AC20 .... connecting and disconnecting under no load.
AC21 .... switching under resistive loads.
AC22 .... switching of mixed resistive and inductive loads.
AC23 .... switching of highly inductive loads.
You may be given 2 ratings on the switch ie. AC22 - 100amps and AC23 - 55AMPs (random guess but shows the reduction when using on inductive loads).

The number of times ive seen so called sparkies fitting say a 40A switch to a machine which is often used to switch off when motor running and assuming 40amp means 40amp regardless and not realising its a AC22 rating - thus isolator fails, welds loses phase etc.
Also to note is their is sometime a suffix at the end either A or B, AC22-A or AC22-B .... the A is for frequent switching and B for infrequent.
Always check the switch rating on its info if you have alot of inductive loads as most switches are sold under their AC22 rating.
 
So a conversation with a fellow spark brought into question what exactly are main switches?

Is a 100A main switch a 100A double pole MCB or does the 100A mean its breaking capacity? I'm sure it's the former but I'm beginning to question my own knowledge as he thought differently.

Also, with different rated RCDs, 25, 40, 63...etc, Does that mean that it will trip if the current passing through it exceeds that amount essentially acting as an RCBO? If so, why do we need RCDs to double up as overcurrent protective devices as well?

This is probably quite a basic question but the reason we wondered is because we all seem to fit CU's with 100A main switches even when the cut out fuse is only rated at 60A or 80A. If the main switch is indeed a 100A overcurrent protective device, why is this?

I find it staggering that an approved electrical contractor would not know these details.
Truely speechless.
 
So a conversation with a fellow spark brought into question what exactly are main switches?

Is a 100A main switch a 100A double pole MCB or does the 100A mean its breaking capacity? I'm sure it's the former but I'm beginning to question my own knowledge as he thought differently.

Also, with different rated RCDs, 25, 40, 63...etc, Does that mean that it will trip if the current passing through it exceeds that amount essentially acting as an RCBO? If so, why do we need RCDs to double up as overcurrent protective devices as well?

This is probably quite a basic question but the reason we wondered is because we all seem to fit CU's with 100A main switches even when the cut out fuse is only rated at 60A or 80A. If the main switch is indeed a 100A overcurrent protective device, why is this?

A main switch provides no overload protection and an RCD also provides no overload protection.

A 100A main switch isnt a double pole MCB nor does it have 100A breaking capacity. An isolator is an OFFLOAD device.

Ok, so in that case, why are there RCDs with different ratings? Why are they not all 100A?

What about for currents over 100A??...


Basics my friend....
 
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I find it staggering that an approved electrical contractor would not know these details.
Truely speechless.

I agree with the above post, frightening that you sign off work

I do apologise guys, it seems I'm in the wrong job! I didn't realise that to be an electrician you had to know everything. Anyway, I'm off, I've got to start job hunting as clearly I'm not cut out for this electrical malarky.

I assure you both that in my new line of work, whatever that may be, I'll be sure to remain ignorant.
 
Erm it isnt About knowing everything , however knowing basics like that is a given

So yes if you didnt know that a main SWITCH is not a breaker you are in the wrong job pal, i take it your another one who doesnt have nvq level 3?
 
vernam616 i think you are being slightly harsh, we all come on here to learn and better ourselves yet you make rash comments? I Think you should go a little easy as maybe one day you might require someones help to a question you are unsure of.
 
Jesus christ get real man, theres no wonder Electrical Trainee get away with it because the world is full of people like you, i can imagine an apprentice asking this sort of question and id gladly answer them, however not an APPROVED CONTRACTOR, boils my blood it really does, someone who signs off work saying its safe is asking whether or not the main switch is a breaker

Your on the same level
 

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