3 phase 125amp or 250amp main switch

swifters · Apr 30, 2013

Why on a 3 phase supply do you only get a choice of 100amp/125amp or 250amp main switches? The basic domestic main switches are normally 100amp. I would have thought that with the added voltage, that there is a greater potential amperage? Granted resistance will be reduced with the 3 phases but is each phase not protected by a 100amp main fuse? 3 x 100amp being 300amps? Why is it that 125amp 3 phase main switches seem to be the norm?

spark 68 · Apr 30, 2013

In the example you posted above re:100A X 3 does not equal 300 A, it equals a 100A per phase.

It could be partly due to the manufacturers rationalising their ranges, and partly due to the need of a greater Icu (kA rating).

swifters · Apr 30, 2013

spark 68 said:
In the example you posted above re:100A X 3 does not equal 300 A, it equals a 100A per phase.

It could be partly due to the manufacturers rationalising their ranges, and partly due to the need of a greater Icu (kA rating).

I'm pretty sure the last job I saw with a 3 phase installation had 3 x 100A fuses. Surely if each phase is drawing 100amp (simultaneously; unlikely I know) then the 100amp main switch would not switch the power off? Would it not break?

spark 68 · Apr 30, 2013

swifters said:
I'm pretty sure the last job I saw with a 3 phase installation had 3 x 100A fuses. Surely if each phase is drawing 100amp (simultaneously; unlikely I know) then the 100amp main switch would not switch the power off? Would it not break?

You are correct wrt having 3 fuses, but they are on separate phases, the switch has 3 poles = 100A per phase in the example you are using.

Hawk81 · Apr 30, 2013

swifters said:
I'm pretty sure the last job I saw with a 3 phase installation had 3 x 100A fuses. Surely if each phase is drawing 100amp (simultaneously; unlikely I know) then the 100amp main switch would not switch the power off? Would it not break?

No main switch would switch the power off due to overcurrent. The fuses are there to protect for over current. A main switch is exactly that a switch.

swifters · Apr 30, 2013

Hawk81 said:
No main switch would switch the power off due to overcurrent. The fuses are there to protect for over current. A main switch is exactly that a switch.

Yes, so if you wanted to switch the power off yourself whilst everything was on then perhaps the main switch would not work? If each pole is capable of switching off 100amps then I suppose no worries? I know an electrician would know that you should switch off the MCB's/RCBO's before switching off the main switch but a layperson wouldn't. They would just flick the main switch, but I suppose the switch is in effect rated at 300amp's in total?

plugsandsparks · Apr 30, 2013

A 100A 3PTP isolater has 3 contacts rated at 100A - it has a physical size appropriate for the size of cable needed for 100A supply. Once above this then say a 250A isolater can be used for anything up to 250A, it will have a bigger physical size to accommodate the larger cables from anything around 25sqmm to 95sqmm odd.
Point is that as long as the isolater is at least as big as the upstream protection, no problems

Engineer54 · Apr 30, 2013

Hawk81 said:
No main switch would switch the power off due to overcurrent. The fuses are there to protect for over current. A main switch is exactly that a switch.

Unless of course the main switch is a MCCB!!

Anything larger than 250A on a typical 3 phase DB would be provided by a MCCB in most instances. Though i have seen more than a few German panels with 400A isolator switches

spark 68 · Apr 30, 2013

Swifters,
Ah! I see what you are getting at, the isolator type you are asking about is usually an off load switch, like you say they are meant to be operated/switched off load (MCB's etc. switched off first), but I don't think the makers uprate them to take this into account, as they are still off load devices.

Outspoken · Apr 30, 2013

Swifters, All Switch Disconnectors must be compliant with the "parent type approval" IEC 69947-2 as well as various individual type approvals from the IEC, and as such they must be able to safely switch the full designed load of the disconnector (I think the requirement is that it must safely switch 150% of the designed load.

Here is some information related to a Merlin-Gerin (Schnieder) Switch Disconnector for one of their TPN panels.

Rating:20 to 125 A
Compliance with standards: EN 60669-1, IEC 669-1 for I 20 to 63 A; IEC 947-3 for I 40 to 125 A
Suitable for isolation in accordance with industrial standards: IEC 60947-2
DC application
Short-circuit withstand:20 x In for 1 s
Number of operating cycles:200 000 for I 20 and 32 A; 50 000 for I 40 to 125 A

So whilst it is advised that all loads should be disconnected prior to switching a main isolator/disconnector, they are actually designed to be capable of doing so.

swifters · Apr 30, 2013

Outspoken said:
Swifters, All Switch Disconnectors must be compliant with the "parent type approval" IEC 69947-2 as well as various individual type approvals from the IEC, and as such they must be able to safely switch the full designed load of the disconnector (I think the requirement is that it must safely switch 150% of the designed load.

Here is some information related to a Merlin-Gerin (Schnieder) Switch Disconnector for one of their TPN panels.

Rating:20 to 125 A

Compliance with standards: EN 60669-1, IEC 669-1 for I 20 to 63 A; IEC 947-3 for I 40 to 125 A

Suitable for isolation in accordance with industrial standards: IEC 60947-2

DC application

Short-circuit withstand:20 x In for 1 s

Number of operating cycles:200 000 for I 20 and 32 A; 50 000 for I 40 to 125 A

So whilst it is advised that all loads should be disconnected prior to switching a main isolator/disconnector, they are actually designed to be capable of doing so.

So in laymans terms it can knacker the switch if you keep turning it on and off under a heavy load.

Engineer54 · May 1, 2013

With the likes of Merlin-Gerin (Schnieder), ABB, Seimens and the like, that's why you pay the big money. They are capable of disconnection under load. That's not to say you make a habit of doing so....