Online assistance for electrical trade people in New Zealand and Australia Login  |  Register  |   Forgot Password
Assistance for electrical trade people




Click here to send Ron a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: Free standing oven - breaker tripping

Nov 09 2018 10:25

Hey guys,

I\'ve installed a 11kw freestandkng oven = 47a. assuming 50% max demand, = 23a.

The breaker is the old lupus, which I have no idea what the over current trip rating is e.g standard MCBs are usually 1.5x for 4 hours etc.

The problem, the customer is using all elements + oven and so drawing the whole 47a on a 35a lupus breaker which operates fairly quickly.

I have stated the options of,
Running another phase to house specifically for two phases to oven, or to not use all elements at once.

Any other suggestions on how to approach this?


Nov 09 2018 10:43

I’ve always struggled with the concept of MD for a dedicated circuit and relying on the breaker to tolerate it for a while in a foreseeable situation. Fine for an installation where you figure everything will not run simultaneously but in this case if the customer wants to use the full load you must be able to provide it.

Is there enough power to the installation to start with (e.g. at the board)? Can you increase the capacity from the board to the oven? This may just shift the problem (or potential for) if the site has other simultaneous loads but the customer must understand the implications of 11Kw of oven.

Is this residential?


Nov 09 2018 11:35

Yeah mate, residential.

I always thought that most ovens are around the 10kw mark, but rarely do they use that continuously e.g thermostats turning on and off etc. But also most homeowners seem to maybe use the oven and 2 elements which usually draws about 30a which most times would be fine.


Nov 09 2018 12:32

Upgrade the MCB to a C32a. You may have an old dicky MCB.

Nov 09 2018 13:04

I agree. Change the breaker.

A modern MCB should hold 1.5x the rating for around a minute and 2x for nearly 10 sec.. Usually that\'s sufficient for diversity of operation of elements on a range.

Also those Lupus breakers don\'t have a very high kA rating.. maybe 3kA?

Generally when installing new equipment, the protective device should be upgraded at the same time.

Nov 09 2018 16:05

The protective device upgrade might be how i look at things going forwed for furture reference,obviously, I inform customers they dont need to, but highly recommended, unless of course its all ceramic rewireable then its a must

Was not an old Lupus either when i went back to sight, was those old PDL with the solid bar going from the main switch to all the MCBs

And i found why it tripped, not overload, a mouse had somehow got under the hob part of oven, short circuited between phase and neutral and dinner was served.

Load test came to 37A with oven + all hobs on high, one of the hobs had a dual ring, with a high function both rings, low function one ring. This only came to 29A so no issues there.

Bloody mice!!
thanks for your guys help, again!

Nov 09 2018 16:50

Seeing that it had operated via a short circuit several times then the MCB should be replaced. New MCBs have a ‘3’ In a square box written on it indicated the max number of shorts before replacement.

Nov 09 2018 18:08

Never heard of 3 short circuits only, or seen that square before. Can\'t say I\'ve looked though.

Nov 09 2018 18:28

Whoops, I\'m thinking of the wrong thing.

Nov 09 2018 20:25

A lot of short circuit protection is only designed to operate once. And by the time it operates,even though the cable of the circuit is protected, other fittings may have been damaged beyond repair. Eg a motor + contactor + overload; unless the fittings are all selected carefully, by the time the short circuit protection operates could be looking at newmcb, contactor & overload, and possibly new motor as well.
That\'s why , in an industrial setting, good practice is to go for Type 2 co-ordination so that all parts can be re-used for the next fault.

Not just industrial; same sort of thing can happen in any field, including domestic. The required protection is for the cable only; and doesn\'t necessarily protect the other fittings (including the one doing the protecting) from damage during the time that the (could be several kA) fault current is flowing). Like a good warrior, an overcurrent device can die in defence of the cable (fuses always do). So yes, if an old mcb has tripped on short circuit several times, it may need to be replaced.

Nov 10 2018 10:55

A observation

An 500 V insulation test on the stove may have found the \"cooked mouse\" on the first service call!

Nov 10 2018 13:56

The protective device upgrade might be how i look at things going forwed for furture reference,obviously, I inform customers they dont need to, but highly recommended, unless of course its all ceramic rewireable then its a must


Nov 11 2018 11:23

Thanks for the information about the circuit protection and operating. I always assumed the protection was good for numerous operations.

That was the first service call,I thought I would ask as I got a call, assuming it was due to an overload as all elements were on at the time.

Asking customer what they were doing eg using all elements on high etc.

Customer had already Reset breaker, nothing wrong, turned up elements and did load test, showed no indication of an overload.
Found that one of there elements was not sitting right, took hob apart, found mouse blown in half so no longer was short circuiting anymore so no problem would have ever been found since mouse was no longer shorting itself.

Sarmajor, WHY what?
Switchboard protection upgrade?
Was just a suggestion from DougP, and a valid one. All new protection alot better at operating and can handle higher fault currents then the old shit ones.
I always suggest if there are kids in the family and why they might want to invest the the newer protection with RCDs. I think the value of kids are greater than a costs of a switchboard upgrade. But again, not legally needed as long as the old stuff is not the ceramic rewireable fuses, than switchboard upgrade a must...or plug in MCBs etc

Nov 11 2018 12:13

I am still having trouble with the insistence that replacement of a SERF is somehow mandated (a must).

Sure if you are replacing the circuit protection then an upgrade to an MCB is an option.

Reg 59 allows any installation or part installation to be maintained or restored to its original condition.

This would allow the use of a SERF if that was the original protection and the PSC was less than 1kA.

You are not allowed to install a SERF in a new installation as per AS/NZS3000:2007 Sections 1.7.2(h), 2.4.3, 2.5.2,

Nov 11 2018 13:10

I think you will find that new stove warrenties require the correct external overcurrent device (an MCB or HRC fuse) to be fitted as there is is no individual element fuses any more.

The internal wiring of the stove has been designed to be protected by the external overcurrent device only. If the external protection does not operate correctly, all of the wiring loom inside the stove will be damaged and you will need to totally replace the loom.

Nov 11 2018 17:20

I have no problem with following the manufacturers recommendations in regards to protection for appliances for warranty protection. Maybe they will incorporate that requirement into the installation rules some time.

I was challenging the replacement because it says somewhere that you just have to do it.

The simple fact is that replacement of SERF’s is not mandated.
Replacement may be required but the electrician should always be able to explain the reasoning and back it up with a black and white reference to a clause from an applicable Standard or Regulation.
I gave the applicable Reg that allows SERF’s to be replaced and also pointed out several references that prohibited them for New work.

That I believe is the whole purpose of this forum. Making people think and have a look in the book to check the references and read it for themselves.

Nov 12 2018 15:05

Good point Sarmajor.
You actually made me re read the section in terms of where it states rewireable fuses not to be used.

2.4.3, 2.5.2
Of course, this does not mean the original install needs to be changed or updated and can remain in service according to the regs.
I only said it is a good suggestion to the homeowner to upgrade and why e.g quicker tripping time, higher KA rating etc.
If I extend circuits I always change the SERFs

Nov 12 2018 16:15

For the sake of your time spent there, a plug in MCB and base are reasonably cheap. Also, SERFs, are very dangerous as short circuit protection.

Nov 12 2018 17:41

SERF’s were perfectly acceptable for a very long time.
Sure if some idiot replaces the fuse wire with bigger wire because it keeps tripping then there will eventually be a problem

If they were really that dangerous the Regulator could simply change the rules to make replacement mandatory.