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#165109 - 06/18/07 01:58 PM Main Switch or Circuit Breaker?
Paulusgnome Offline
Member

Registered: 06/15/04
Posts: 56
Loc: Christchurch, New Zealand
I have a question that I would like to get some feed back on.

Here in New Zealand, it is standard practice, and endorsed by the regulations, to wire up domestic switchboards using a main isolating switch as opposed to using a circuit breaker, relying on the presence of pole fuses to protect the switchboard. I am inclined to the view that, for the very modest extra cost, using a main MCB instead of an isolator would be quite a sensible practice. Admittedly, switchboard faults are rare, but the thought of a whole switchboard being destroyed because of a fault that a main breaker could have interrupted concerns me just a bit. Also, it is not totally unheard of for pole fuses to be either overrated or to have been mistakenly replaced with solid links and at least a main breaker would provide insurance against this.

Comments, anyone?


Edited by Paulusgnome (06/18/07 01:59 PM)
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#165125 - 06/19/07 04:55 AM Re: Main Switch or Circuit Breaker? [Re: Paulusgnome]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
Having grown up with a plethora of cascaded main fuses I absolutely second that! If those fuses are located ahead of the board it also facilitates replacing the entire board because you don't have to get the PoCo to pull the pole fuses.

The extreme are apartment buildings that are set back from the street. One set of fuses (e.g. 160A) at the service entrance at the street (pillar). Then where the service enters the house (100A). Then each apartment got individual fuses (20 or 25A) ahead of the meter. Single family homes leave out the second set of fuses.

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#165130 - 06/19/07 09:51 AM Re: Main Switch or Circuit Breaker? [Re: Texas_Ranger]
mikesh Offline
Member

Registered: 06/07/06
Posts: 614
Loc: Victoria, BC, Canada
Wow I don't think I believe that one. No OC device at the house? Do you really mean an isolating switch (no fuses) is all the service requires?
There was a time here where some houses had no main and I know of two of those that burned through the enclosure. Fortuneately the houses did not burn down. A Utility transformer is often so overfused that the transformer will feed a fault for a long time before anything blows. Sounds like there are a few things going on down under that have me scratching my head.

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#165170 - 06/21/07 04:28 AM Re: Main Switch or Circuit Breaker? [Re: mikesh]
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
The standard arrangement here is for a main switch on the board, not a circuit breaker. TT installations will have a main RCD, but that does not incorporate overload protection.

It is the norm, however, for a PoCo cartridge fuse to be fitted alongside and wired ahead of the meter.

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#165180 - 06/21/07 06:55 PM Re: Main Switch or Circuit Breaker? [Re: pauluk]
wa2ise Offline
Member

Registered: 11/29/02
Posts: 771
Loc: Oradell NJ USA
Does each customer get his own fuse? Up on the pole if not at the meter?

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#165188 - 06/22/07 01:51 AM Re: Main Switch or Circuit Breaker? [Re: wa2ise]
Paulusgnome Offline
Member

Registered: 06/15/04
Posts: 56
Loc: Christchurch, New Zealand
Yes, each customer gets his own fuse. This is generally either up a pole, or if the lead-in is underground it is in a plastic pillar out on the property boundary. I'm not that expert at this side of things though - others such as Trumpy or Rodalco can supply a lot more detail about this aspect. Guys?
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#165239 - 06/23/07 05:45 PM Re: Main Switch or Circuit Breaker? [Re: Paulusgnome]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Interesting thread Mark,
I've often wondered this myself.
For years here, houses have been using the traditional 2 pole main swith on the switch-board, one pole isolating the mains supply to the house, the other isolating the Hot-water supply.
However, we must not get confused with the service line fuse here, that is only designed to protect the service line itself, not what is on the end of it, the mere fact that it often does, makes people think that that is what it is installed for.
For years, a pole fuse was nothing more than a piece of 63A rated tinned copper wire, indeed, the NZI pole fuse holders were made for this.
Nowadays, where overhead reticulation is still used (and this is getting less and less every year), most pole fuses are slowly being upgraded to BS88 HRC fuses.
Now, when a place is underground reticulated, HRC fusing is used as standard:
Refer the pic below:



Mark,
I really can't see why an MCB can't be used a s a Main Switch here.
In any well designed installation, the MCB (as the Main Switch) should never trip anyway.
Provided that the lower rated sub-circuits are co-ordinated with the I2t characteristics of the Main MCB, there should never be a trip.
From an electrical safety point of view, this idea is a great one.
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#165260 - 06/24/07 03:46 AM Re: Main Switch or Circuit Breaker? [Re: Trumpy]
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Originally Posted By: Trumpy
For years here, houses have been using the traditional 2 pole main swith on the switch-board, one pole isolating the mains supply to the house, the other isolating the Hot-water supply.


2-pole main switches here as well, but we open both line and neutral with them. Where a separate night-tariff is supplied via contactor, there will be a completely separate double-pole main switch for that as well.

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#165413 - 06/27/07 04:46 AM Re: Main Switch or Circuit Breaker? [Re: pauluk]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
Germany largely specifies E type selective MCBs for services now, they're guaranteed to trip before a fuse ahead of it blows, so in case of an overload the customer can restore power without involving the PoCo. Those SLS as they're called are usually located in the meter enclosure, ahead of the meter.

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#165644 - 07/02/07 04:11 AM Re: Main Switch or Circuit Breaker? [Re: Texas_Ranger]
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
The type E circuit breaker is a bit different from standard MCB's: It will trip in a much narrower range than type B or C, making it more selective. Type E trips between 1.05 - 1.2 times rated current, compared to 1.13 - 1.45 times for types B and C.

Maybe this performance will filter down to new MCB's in a few years.

See this PDF from ABB:

http://tinyurl.com/32hx2z

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