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Posted By: Paulusgnome Main Switch or Circuit Breaker? - 06/18/07 08:58 PM
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?
Posted By: Texas_Ranger Re: Main Switch or Circuit Breaker? - 06/19/07 11:55 AM
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.
Posted By: mikesh Re: Main Switch or Circuit Breaker? - 06/19/07 04:51 PM
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.
Posted By: pauluk Re: Main Switch or Circuit Breaker? - 06/21/07 11:28 AM
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.
Posted By: wa2ise Re: Main Switch or Circuit Breaker? - 06/22/07 01:55 AM
Does each customer get his own fuse? Up on the pole if not at the meter?
Posted By: Paulusgnome Re: Main Switch or Circuit Breaker? - 06/22/07 08:51 AM
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?
Posted By: Trumpy Re: Main Switch or Circuit Breaker? - 06/24/07 12:45 AM
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:

[Linked Image]

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.

Posted By: pauluk Re: Main Switch or Circuit Breaker? - 06/24/07 10:46 AM
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.
Posted By: Texas_Ranger Re: Main Switch or Circuit Breaker? - 06/27/07 11:46 AM
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.
Posted By: C-H Re: Main Switch or Circuit Breaker? - 07/02/07 11:11 AM
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
Posted By: djk Re: Main Switch or Circuit Breaker? - 07/03/07 04:01 PM
Irish distribution boards all have a main switch and main fuse on the board itself. This has been standard practice going right back to the early days.

This was originally a diazed fuse, now more commonly a minized switch-fuse unit that looks like this:

[Linked Image from detech-shop.de]

[Linked Image from automation.siemens.com]

They carry a standard neozed fuse [Linked Image from automation.siemens.com]

There's also a sealed double-pole switch after the meter on more modern installations. (Sealed by the contractor after certification of the system)

The logic being, that on a 230/400V distribution system the end users couldn't realistically expect fault protection from the network side. It would take a pretty serious distribution system fault for the over current protection on a pad mounted transformer to kick in.


Posted By: Trumpy Re: Main Switch or Circuit Breaker? - 07/06/07 08:27 AM
C-H,
Are you saying that Type E will trip quicker than either Type B or C curve MCB's?
Reason I ask this is because we use B curve breakers here for standard domestic installations here, C curve is used in places like commercial installations, where there might be a bit of inductive loading or for Submain cables to an out-building.
D-curve is used exclusively for motor protection.
Now any type of E-curve breaker (never having heard of it before and with it being a Selective type also) makes me think that it could be a heavier type, not unlike our Selective RCD's that have a trip level of 300mA.
Your thoughts?.
Posted By: C-H Re: Main Switch or Circuit Breaker? - 07/06/07 07:08 PM
Mike,

yes that is basically what I'm saying. (Darn, it is near impossible to type on a german kezbaoard which has z and y transposed)

As I understand it, it is supposed to keep the utilitys fuse on the pole from blowing. This saves the utility some work in replacing fuses, I suppose. It also allows the consumer to overload the breaker less than with a type B or C, which I suppose translates into less load on the network. As the main breaker is anyway bigger than the rest of the breakers in the house, it should still be selective towards smaller breakers. (E.g. the main breaker is 63 A and the outlets are on 20A circuits.)

(I am in Switzerland at the moment on a business trip. All outdoor cable here is orange. Not rubber, but something else. I swear that one in every ten vehicles this morning belonged to a sparky and there were guys at work everywhere. Later in the day, lots of party tents with beer halls and shops went up later, explaining where the cables were to go.)
Posted By: briselec Re: Main Switch or Circuit Breaker? - 07/17/07 02:17 PM
From the Draft for Public Comment for AS3000-
2.5.1.1 Consumers' mains
Overcurrent protection of consumers' mains shall be arranged as follows:
(a) Short circuit protection and overload protection shall be provided at the origin of the
consumers' mains (the point of supply); or
(b) Short circuit protection shall be provided at the origin of the consumers' mains and
overload protection shall be provided at the main switchboard; or
(c) Short circuit protection need not be provided where -
(i) Overload protection is provided at the main switchboard; and
(ii) The consumers' mains are constructed and installed in such a manner as to
minimise the risk of short circuit.
Notes
1. Single core insulated and sheathed cables enclosed in Heavy Duty PVC conduit to
AS/NZS 2053 are deemed to satisfy this provision.
2. Underground wiring or fire rated wiring enclosures may also be used to satisfy this
provision.
3. An electricity distributor's service protective device may provide short circuit protection for
consumers' mains.
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