Changing Oven Isolation Switch

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Paul1977

5074 posts

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#249191 30-Apr-2019 09:33

Another question for the electricians out there. I'm pretty confident I know the answer, but since I'm not an electrician I figure it's prudent to confirm before proceeding.

Note, I am the home owner and so am legally allowed to change switches etc.

We just repainted our kitchen and so want to update the old 1960's style switches with more modern PDL 600 series ones. Standard power points and light switches have been no problem as they are simple like-for-like replacements.

I assumed the oven isolation switch would be a like for like as well, but when I took the old switch off the wiring doesn't match the replacement. I'll describe it and provide pictures for clarification.

The oven is on a 250V 30A fuse on the switchboard. The old switch itself is far beefier and is rated for 500V at 60A. The cable entering and exiting the switch has phase sheathed in red, neutral sheathed in black, and earth is unsheathed; then the whole lot is sheathed in white. Old switch has 4 terminals - 2 phase, 1 neutral, 1 earth. Phase in and phase out are each wired to the phase terminals (1 in each), both neutrals are twisted together and connected to the neutral terminal, the earths are twisted together then sheathed in green for a short length before being connected to the earth terminal.

I tested the switch with a multimeter and the only switching that is happening is between the 2 phase terminals, so my assumption is that the neutral and phase terminals and just loop terminals.

The replacement switch is rated 250V 32A and has 2 phase terminals, but no neutral or earth terminals.

My conclusion:

Since the oven is on a 30A fuse, then the replacement 32A switch is sufficient (even though it is replacing a 60A switch).

Connect phase in and phase out to the 2 phase terminals on the new switch.

Put a screw connector over the end if the two twisted neutrals.

Cut the exposed wire of the end of the sheathed section of the earth wires, and put a screw connector over the end for good measure.

Is this all correct?

Click the below pictures for full size:

Fuse:

Old switch front:

Old switch back:

Wires:

Wire size (pencil for scale):

New switch front:

New switch back:

Thanks

Paul

Home: Work:
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DarthKermit

5346 posts

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Trusted

#2227786 30-Apr-2019 09:59

That should all work fine. You may want to consider replacing the metal flushbox in the wall with a mordern plastic one to give you more room to work with.

PDL also do a different range switch with a much larger rocker switch. It comes in single and two phase versions.

Paul1977

5074 posts

Uber Geek

#2227825 30-Apr-2019 10:44

DarthKermit:

That should all work fine. You may want to consider replacing the metal flushbox in the wall with a mordern plastic one to give you more room to work with.

PDL also do a different range switch with a much larger rocker switch. It comes in single and two phase versions.

Thanks. I do actually like the look of the larger rocker, but I told the supplier it was on a 30A fuse so the gave me the smaller 32A switch (the larger is 45A). I might swap it depending on cost, although the brochure only lists the larger one as double pole, I can't see a single pole option.

Paul1977

5074 posts

Uber Geek

#2227835 30-Apr-2019 10:53

I guess the next question is:

Should I be using a double pole switch anyway with phase on one pole and neutral on the other, so as to be able to truly isolate the oven (i.e. switch in "Off" position disconnects phase AND neutral)?

Aredwood

3885 posts

Uber Geek

#2227876 30-Apr-2019 11:22

Paul1977:
I guess the next question is:

Should I be using a double pole switch anyway with phase on one pole and neutral on the other, so as to be able to truly isolate the oven (i.e. switch in "Off" position disconnects phase AND neutral)?

Just stick with a single pole switch. As using a double pole switch would be a modification. And can also cause it's own set of problems should the neutral side fail and the phase side keep on working. In that the stove would appear off when in reality its wiring would still be live.

If you are worried about safety, get an electrician to install the new switch. And get him to replace that old fuse on the switchboard while he is there. As those fuses have a terrible fusing factor. You would probably blow the pole fuse that controls power to your whole house before that old stove fuse would blow. And if your stove is relatively modern. Its installation instructions would probably require a modern MCB to be installed. As that old fuse wont be able to protect the internal wiring in the stove.

And it wont be much fun if an element fails short circuit, and takes out power to the whole house. Rather than just the stove.

Paul1977

5074 posts

Uber Geek

#2227935 30-Apr-2019 12:34

Aredwood:
Paul1977:

I guess the next question is:

Should I be using a double pole switch anyway with phase on one pole and neutral on the other, so as to be able to truly isolate the oven (i.e. switch in "Off" position disconnects phase AND neutral)?

Just stick with a single pole switch. As using a double pole switch would be a modification. And can also cause it's own set of problems should the neutral side fail and the phase side keep on working. In that the stove would appear off when in reality its wiring would still be live.

If you are worried about safety, get an electrician to install the new switch. And get him to replace that old fuse on the switchboard while he is there. As those fuses have a terrible fusing factor. You would probably blow the pole fuse that controls power to your whole house before that old stove fuse would blow. And if your stove is relatively modern. Its installation instructions would probably require a modern MCB to be installed. As that old fuse wont be able to protect the internal wiring in the stove.

And it wont be much fun if an element fails short circuit, and takes out power to the whole house. Rather than just the stove.

Thanks for that @Aredwood. Not concerned about safety, I had just read that sometimes isolation switches are setup that way. I'm happy to stick with single pole so it's not a modification.

I do like the larger rocker switch on the double pole switch though, presumably it wouldn't be considered a modification if I get the double pole switch but only use one pole? I.e. use one pole for phase, do the neutral and earth with screw connectors, and leave the second pole unconnected?

Next time we need an electrician I'll have him replace the fuse, but don't want a visit just for that so will proceed with just the switch for now.

Thanks

BruceHamilton

77 posts

Master Geek

#2228069 30-Apr-2019 16:30

Just a mention that an electrical inspector suggested we move the Oven Isolation switch whilst checking the electrical safety of the house. His concern was that users would have to reach over the cooktop ( and any hot pots ) to get to it. Apparently a lot of older houses had it located there, whilst newer ones had it located in a safer position.

elpenguino

3431 posts

Uber Geek

#2228094 30-Apr-2019 17:30

BruceHamilton:

Just a mention that an electrical inspector suggested we move the Oven Isolation switch whilst checking the electrical safety of the house. His concern was that users would have to reach over the cooktop ( and any hot pots ) to get to it. Apparently a lot of older houses had it located there, whilst newer ones had it located in a safer position.

As per @Aredwood's info, no, neutrals don't get switched as a rule.

Have you read ECP 51?

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Paul1977

5074 posts

Uber Geek

#2228165 30-Apr-2019 20:08

BruceHamilton:
Just a mention that an electrical inspector suggested we move the Oven Isolation switch whilst checking the electrical safety of the house. His concern was that users would have to reach over the cooktop ( and any hot pots ) to get to it. Apparently a lot of older houses had it located there, whilst newer ones had it located in a safer position.

I was just reading about that myself. The position of our is probably borderline.

Paul1977

5074 posts

Uber Geek

#2228182 30-Apr-2019 20:26

elpenguino:
As per @Aredwood's info, no, neutrals don't get switched as a rule.

Have you read ECP 51?

Yeah, I looked through that this afternoon and saw the bit about that.

I ended up getting the double pole switch as I liked the look of the big rocker, but I wired it as single pole (i.e. phase on pole 1 and nothing on pole 2, then screw connectors for neutral and earth).

EDIT: I might put a label behind the face plate saying it is a double pole switch that is only using one pole so there’s no confusion down the road if someone else needs to do something with it.

andrewNZ

2487 posts

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Inactive user

#2228228 30-Apr-2019 20:55

Although we usually only switch the phase, there is nothing "wrong" with switching both phase AND neutral together, a lot of appliances do it. You must use a double pole switch though.
You would NEVER switch only the neutral, that's dangerous with our electrical system.

A label is unnecessary. Frankly, if it confuses someone, they shouldn't be doing it.

Aredwood

3885 posts

Uber Geek

#2228243 30-Apr-2019 21:32

andrewNZ: Although we usually only switch the phase, there is nothing "wrong" with switching both phase AND neutral together, a lot of appliances do it. You must use a double pole switch though.
You would NEVER switch only the neutral, that's dangerous with our electrical system.

A label is unnecessary. Frankly, if it confuses someone, they shouldn't be doing it.

Appliances are a completely different situation. As some places have 230V delta or split phase power. Meaning that the power sockets have 2 live pins and an earth pin. Alot of cheap generators in NZ also supply 230V phase - phase. And there is also more of a risk with miswired plugs etc of the phase and neutral being reversed to an appliance.