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confused: main switch vs circuit breaker?

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  1. #1
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    Question confused: main switch vs circuit breaker?

    Hi.

    I'm a bit confused about how the main switch works on our electrical switch board. Wondering if someone can help clarify for me?

    Currently I have a Hager SB 180T main switch.

    Theoretically, what happens if my load exceeds 80A? Will this switch just trip and turn off, and I just have to turn it on again? Does it work like a breaker? Or will it blow or fuse and have to be replaced? Or something else?

    What is the difference between a main switch and a normal circuit breaker (other than the ampage rating)?

    BTW I'm not planning on changing any switches myself (if necessary, I would get an electrician to do that), just trying to understand how it works.

    Thanks!

  2. #2
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    This unit would be an isolating switch (Load break switch) only and not a circuit breaker. A circuit breaker is designed to automatically open the circuit under an overload or short circuit situation. An isolating switch is a manually operated device to open and close the circuit and will not switch off automatically under overload situation. Some isolating switches are designed to be manually opened or closed on low load only. Where full load is to be switched on or off it would be recommended to use a Load Break Switch. I think the SB180T is classed as a load break switch designed to switch 80A load current.

    In the unlikely event tha your load exceeds 80A and no fault conditions apply then it will rely on the service fuse to protect the consumers mains. Once the rating of the service fuse is exceeded then it will break the circuit.
    Juan


    "If the enemy is in range, so are you."

  3. #3
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    Herewith "Electrical Installations 101"

    The Main Switch is just that. The "Big Daddy" switch that turns the power off to the whole of the electrical installation. The rating of the Main Switch refers to how much current it can switch. It normally doesn't have any overload protection capabilities. The Supply Authority will have their fuses between the consumers mains (the wires coming into your place) and the Main Switch as protection against catastrophic overload.

    After the Main Switch, power is distributed to the various circuits (lights, power, air conditioning, hot water etc) which are each protected by fuses, or more commonly these days by circuit breakers or combined circuit breaker/RCDs (residual current devices or "safety switches").

    The circuit breakers can be turned off to isolate a circuit and will also automatically isolate the circuit if an overload or short circuit occurs. They are rated to suit the expected circuit load and size of cable used. If the rated current is exceeded, then the circuit breaker will "trip". This isn't uncommon in winter when people plug in too many heaters to one circuit and hence overload it.

    RCDs are a further level of protection that is required for power and lighting in new installations and recommended as a retrofit for old installations. The RCD senses any small current inbalance between the circuit active and neutral. This indicates that current is flowing to earth somewhere (possibly through you!!!) and disconnects the supply to the circuit before any damage is done or electrocution injury takes place. They are not normally connected to circuits such as stoves or hot water, where there can be some inherent leakage current in elements which might cause false "tripping".

    These days, one of the issues to be aware of with Main Switches is that they only disconnect the "mains supply". They don't disconnect the supply from a "Solar Generator" which also has to be disconnected separately.

  4. #4
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    Thanks for the replies. So with an 80A Load Break Switch, if the load exceeds 80A, and the fuse at the street is larger than 80A, what would happen? Will the LBS fuse open? Will some wire burn-out somewhere in between, possibly catching on fire..?

    Can the 80A LBS be replaced with an 80A circuit breaker? Are 80A CB even available? (I have seen 63A but not 80A)

    If the fuse blows on the street, do you have to get supply company out to fix it?

    Also, what is the general rating of cable thickness to current-load capacity, for different thicknesses of the main supply?

    Thanks again!

  5. #5
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    Quote Originally Posted by craigj-au View Post
    Thanks for the replies. So with an 80A Load Break Switch, if the load exceeds 80A, and the fuse at the street is larger than 80A, what would happen? Will the LBS fuse open? Will some wire burn-out somewhere in between, possibly catching on fire..?

    For a standared domestic installation you will find that the supply fuses will be in your switchboard(or off to one side) not pole fuses and you will generally find that supply fuses are sized the same as the main switch in your switchboard.

    Quote Originally Posted by craigj-au View Post
    Can the 80A LBS be replaced with an 80A circuit breaker? Are 80A CB even available? (I have seen 63A but not 80A)
    Yes but a main switch will do fine.


    Quote Originally Posted by craigj-au View Post
    If the fuse blows on the street, do you have to get supply company out to fix it?
    Yes

    Quote Originally Posted by craigj-au View Post
    Also, what is the general rating of cable thickness to current-load capacity, for different thicknesses of the main supply?

    Thanks again!
    There are many are many variables to consider. Such as installation conditions(how it's installed), insulation temperature rating, conductor material. There's no straight forward answer to that question............

  6. #6
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    Maybe I should give a bit of background.. We have been doing some major renovations, and have added/will add heaps of circuits, and there is potential (although it shouldn't be common) for the load to exceed the 80A. I'm pretty sure the supply fuse is on the pole. I'm not really sure of the rating of the fuse, or the thickness of cable beteween that fuse and my main switch.

    Basically I don't really want the situation where some fuse blows and we have no electricity until someone can come and fix it. I wasn't sure if the mains switch was supposed to trip or not, or if it could be replaced with a circuit breaker. It seemed like a safer option to install a circuit breaker instead of the main switch. If it so happened that we exceeded 80A I figured that the CB should trip, and then I can turn some things off, and reset the CB. On a quick look I could only find 63A breakers and not 80A ones, and didn't really want to "downgrade". Also don't really want to have to rewire the whole supply line to a higher rating (doubt we would generally exceed the 80A).

    Hope that helps understand my questions. I didn't just want to give my situation originally, as I wanted to properly understand how it all works, not just get the answer for my situation.

  7. #7
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    Quote Originally Posted by Hitchy View Post
    For a standared domestic installation you will find that the supply fuses will be in your switchboard(or off to one side) not pole fuses and you will generally find that supply fuses are sized the same as the main switch in your switchboard.
    This must vary between states. Here in Tas the service fuse is always out on the street - either in a turret or up the pole (for a domestic installation). Only commercial installations (or 3 phase domestic) would have fuses on the switchboard.

    For new installations here there's also a supply authority main switch before the meters, in addition to the normal main switch or circuit breaker for each tariff. Older homes will just have a main switch or breaker for each tariff, but not a big one for the whole lot before the meters.

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    Quote Originally Posted by Smurf View Post
    This must vary between states. Here in Tas the service fuse is always out on the street - either in a turret or up the pole (for a domestic installation). Only commercial installations (or 3 phase domestic) would have fuses on the switchboard.

    For new installations here there's also a supply authority main switch before the meters, in addition to the normal main switch or circuit breaker for each tariff. Older homes will just have a main switch or breaker for each tariff, but not a big one for the whole lot before the
    meters.
    In Queensland we have one of these where the service terminates before going to the main switch. They are sealed by the supply authority the service fuse can be on the fascia or pole.

    Forgot the link to the link Service Link 500V 100A Clear Sparky Direct - Online Electrical Wholesaler- Electrical Accessories

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    Quote Originally Posted by craigj-au View Post
    Maybe I should give a bit of background.. We have been doing some major renovations, and have added/will add heaps of circuits, and there is potential (although it shouldn't be common) for the load to exceed the 80A. I'm pretty sure the supply fuse is on the pole. I'm not really sure of the rating of the fuse, or the thickness of cable beteween that fuse and my main switch.
    If it is an ordinary home you have nothing to worry about as you could have many circuits and outlets but you will not use them all at one time that's how the maximum demand is calculated.

  10. #10
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    Here in SA we also require a "meter isolator" before the meters (so the power supply company can send some bunny unskilled person to lock off your power if you don't pay your bill) rather than pulling the supply fuses.
    On my new supply I have the meter isolator which is a circuit breaker, the main switch (another circuit breaker) and then two more main switches (also circuit breakers) for the branch to the shed and to the house. All "switches" are 63A three phase circuit breakers rather than isolating switches because the cost of a switch was 2-3 times the cost of a circuit breaker!! Crazy supply and demand / mass production?

    Your main supply cable should be appropriately rated for the protection fuse supplying your switchboard so there should be no problem with the cables failing.

    There is no reason why you (your electrician) couldn't put in an appropriately graded circuit breaker as your main switch and avoid the possibility of blowing your supply fuse but is is not a likely scenario unless you draw some serious current.

    Supply fuses can be on the pole, at the point of supply on the house or in the meter box. It probably depends on your state, the type of supply and when your supply was installed.

  11. #11
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    Quote Originally Posted by Bros View Post
    If it is an ordinary home you have nothing to worry about as you could have many circuits and outlets but you will not use them all at one time that's how the maximum demand is calculated.
    Hmm, not sure what defines "ordinary" In the end, we will probably have 3x ovens, 2x cooktops, 2x washing machines, 1x dryer, 2x dishwasher, 1x electric hw booster, plus possibility of things like aircons, frypans, irons etc, mostly on different circuits. So while it should be unlikely, it is possible the sum of heavily loaded circuits could exceed 80A at one given time (eg sometimes we have a lot of people over and could be lots of cooking going on).

    BTW is Clipsal 4CB180/10 the right kind of 80A breaker to replace a main switch? (Closest thing I could find on the Clipsal site)

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    No it is not an ordinary house.

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    As Bros says, circuit cable and protection is determined by calculating "Maximum Demand". AS3000 Appendix C goes into this in some detail, see if you can get hold of a copy. Maximum Demand takes into account that not all things connected to a circuit will be on at once. Even with stoves, unless all the knobs are wound up "flat-chat" the thermostat controls will have elements on and off at different times in order to control the average heat.

    With properly rated circuit breakers on each circuit, the Main Switch/Service Fuses should never be affected if there is an overload. Catastrophic short circuits are another thing, but if that happens there will be plenty of other stuff to fix as well.

    As others have said, you could fit a Main Switch/CB that is rated at less than the service fuses if you think there might be a problem, but practically that is unlikely and would suggest that the circuits/supply need re-designing.

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    Alright I'll say it - his licensed electrician doing the work will know all this and comply with the relevant standards - and will do the paperwork so all the work done for the reno is approved (making the insurers, banks and everyone else comfortable. If he hasn't done the right thing then report him to your power company or to the Qld regulator Don't do your own electrical work - Department of Justice and Attorney-General
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  15. #15
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    Quote Originally Posted by Bloss View Post
    Alright I'll say it - his licensed electrician doing the work will know all this and comply with the relevant standards - and will do the paperwork so all the work done for the reno is approved (making the insurers, banks and everyone else comfortable. If he hasn't done the right thing then report him to your power company or to the Qld regulator Don't do your own electrical work - Department of Justice and Attorney-General
    Hi.

    What paperwork does the electrician need to do? I didn't realise they had to give special paperwork. I just got Tax Invoices for work done. The electrician suggested 63A circuit breaker next time he came out, but I'm thinking maybe 80A better. (I'm in IT and can sometimes buy stuff from my wholesalers, but I get him to install it. Also, I think he doesn't usually get involved in more complicated stuff anymore, but I'd rather keep using him if I can rather than get a different electrician.)

    Also, I spoke with an electricial engineer who seemed somewhat familiar with the standards, and he seemed to think it would be ok to have a lower CB instead of higher rated infrastructure. Do you know if that is correct or not?

    Thanks!

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    I don't know about Qld but NSW electricians were self certifying unless it changed & power suppliers will only guarantee 63 amps at the meter board in a normal domestic situation

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    I don't see any problem with anyone trying to understand the in-and-outs of their electrical installation. The OP has asked some good questions.

    I'll offer the following comments:


    1. The standard uses the term 'maximum demand'. This is very poor terminology IMHO. It is just a term to describe the rating of the installation which could be exceeded in most installations if one really wanted to - and pop the supply fuse. i.e. your maximum demand could exceed the rating. The termology should be something like 'maximium likley demand', 'maximium reasonable demand' or 'maximium typical demand'.
    2. The standard offers several methods of determining 'maximum demand': Calculation, Assessment, Measurement and Limitation.
    3. Replacing the main switch with a circuit breaker is an example of the 'limitation' method.
    4. The standard offers a guide for calculating 'maximum demand' that assume certain 'diversity factors'. These factors may or may not be appropriate for a particular installation.
    5. The section of the standard with the 'after diversity maximum demand' calculation is an 'informative' appendix and should be treated as such - informative, but it may not be applicable in particular circumstances.
    No laurels to rest on

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    Electrical Safety Act Regulation Pg 24
    15 Certificate of testing and safety(1) This section applies if a licensed electrical contractor
    performs electrical work that must be tested under this
    division.[s 16]
    Electrical Safety Regulation 2002
    Part 2 Electrical work
    Reprint 4E effective 1 January 2012 Page 25
    (2) The contractor must, as soon as practicable after the testing,
    ensure that the person for whom the electrical work was
    performed is given a certificate complying with this section.
    Maximum penalty—40 penalty units.
    (3) The certificate must state the following—
    (a) the name and address of the person for whom the work
    was performed;
    (b) the electrical equipment tested;
    (c) the day the electrical equipment was tested;
    (d) the number of the electrical contractor licence under
    which the electrical equipment was tested.
    (4) The certificate must certify that the electrical equipment, to
    the extent it is affected by the electrical work, is electrically
    safe.
    (5) A licensed electrical contractor must keep a copy of a
    certificate given under this section for at least 5 years after the
    certificate is given.
    Maximum penalty for subsection (5)—20 penalty units.

  19. #19
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    Quote Originally Posted by Bros View Post
    Electrical Safety Act Regulation Pg 24
    I just looked at one of my invoices and it says "I certify that the electrical installation, to the extent it is affected by the electrical owrk, has been tested to ensure that it is electrically safe and it is in accordance with the requirements of the wiring rules and any other standard applying to the electrical installation of the electricity saftey regulations of 2002" I guess that is the "paperwork" as per Regulation excerpt..?

  20. #20
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    Pays to read the fine print.

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