100A breaker?

[D Skelton]

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?

 

[Amp David]

A 100 amp main is a switch. Its rated to carry and be switch whilst carrying 100 amps

 

[telectrix]

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.

 

[D Skelton]

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

 

[Amp David]

Cost?????

 

[D Skelton]

Hmm, so you reckon it's as simple as that? Makes sense I suppose.

Cheers chaps

 

[Amp David]

You're welcome, invoice is in the post:stuart:

 

[mogga]

You're welcome, invoice is in the post:stuart:

obviously not up to speed with the P.O. price rises Email ios cheaper these days

 

[D Skelton]

You're welcome, invoice is in the post:stuart:

I pay cash only I'm afraid. Feel free to come down and collect it however

 

[Amp David]

That's fine chief

 

[baldsparkies]

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.

 

[Darkwood]

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.

 

[Guest55]

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.

 

[vernam616]

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

 

[ipf]

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

Same reason switches are not all 100A.