Unobtrusive Isolators close to Ovens but needing very long cable runs, are such runs OK.

[Nickfromwales]

A robot in cyberspace says this:

 

13

 

 

 

A latching contactor relay (or bi-stable relay) maintains its state without continuously powered coils. A momentary pulse "sets" the contacts closed, while a reverse polarity pulse or a separate mechanical/electrical reset pulse "resets" them to the open position. This saves significant energy and prevents buzzing or drop-out during power dips. [1, 2, 3, 4, 5]

 

Why Latching Contactors are Used

Standard relays and contactors are "fail-safe," meaning they require constant power to hold a circuit closed. Latching contactors are highly beneficial in specific applications: [1]

• Energy Efficiency: Ideal for systems that stay on for long durations, as no power is consumed to keep the contacts engaged. [1, 2]

[Mattg4321]

Or get a normally closed contactor. 

[Nickfromwales]

1 hour ago, Mattg4321 said:

Or get a normally closed contactor. 

I have some odd, inbuilt aversion to this needing any kind of constant (continuous / maintained) signal or feed to function, and am favouring something that flip / flops with a single ‘hit’.

 

I may need therapy……but I do also like belts, braces, and for my phone to continue to remain silent each and every night.

[Mattg4321]

7 hours ago, Nickfromwales said:

I have some odd, inbuilt aversion to this needing any kind of constant (continuous / maintained) signal or feed to function, and am favouring something that flip / flops with a single ‘hit’.

 

I may need therapy……but I do also like belts, braces, and for my phone to continue to remain silent each and every night.

It’s a fair point if you want it to

go off and stay off in the event of the smoke detector circuit going down. 

[Nickfromwales]

2 hours ago, Mattg4321 said:

It’s a fair point if you want it to

go off and stay off in the event of the smoke detector circuit going down. 

Madness I know (IRL) and that was my exact concern.

 

If I can buy one thing to be a maybe, or another that’s a definite, and the price isn’t hundreds apart, then I’ll go definite each and every time.

[joth]

On 07/07/2026 at 23:16, Nickfromwales said:

Criticism / objections (or applause) welcomed

Aside from the whacky surprise it leaves to any future electrician working on it, I'd be concerned fire alarms come with a "test monthly" sticker or similar, and this thing frobbing off so regularly will decrease lifetime of the rcbo and relay base, and increases odds it gets forgotten and not re-enabled

 

 

[Nickfromwales]

3 hours ago, joth said:

Aside from the whacky surprise it leaves to any future electrician working on it, I'd be concerned fire alarms come with a "test monthly" sticker or similar, and this thing frobbing off so regularly will decrease lifetime of the rcbo and relay base, and increases odds it gets forgotten and not re-enabled

 

 

The benefit outweighs these ‘odds’, as the MVHR not supplying a fire with a plentiful feed of fresh air (to aid combustion) would be my preference; the significance of this was enough for me to consider it robustly, pros and cons, before suggesting it to my client.

 

Any future electrician would be informed by the owner, before they took any tools out, and would also encounter printed labels on the CU stating the additional measures required. 
 

However, if that all failed, the absolute worst case scenario THEN, would be the owner eventually realising that the lMVHR wasn’t on then they’d simply go and reset it.

 

Number of people who test their smoke detectors monthly?  . Not so sure but I’d bet that It’s a VERY low percentile…..  …..like single digits.

 

Only the relay would wear, not the RCBO🙂, and this will likely be tested and approved for 1000’s of operations. In actuality, it’ll fire maybe 2-5 times per decade.

[Spinny]

Can MVHR be used to distribute gaseous fire suppression ?

[JohnMo]

Why is MVHR running any different from open trickle vents when you have a fire. The fire consumes as much oxygen as it can, the fire becomes a low pressure zone, sucking air via opening anyway. Running or not it will suck air through the MVHR or from other rooms via the plenum. If you want MVHR to not contribute to fire air supply you need a fire dampers also.

[Nickfromwales]

1 hour ago, Spinny said:

Can MVHR be used to distribute gaseous fire suppression ?

Nope!! B Regs have an aversion to death inducing systems being installed in residential dwellings lol.

 

Waking up to a home being fully filled with Halon would not go well……

[Nickfromwales]

1 hour ago, JohnMo said:

Why is MVHR running any different from open trickle vents when you have a fire. The fire consumes as much oxygen as it can, the fire becomes a low pressure zone, sucking air via opening anyway. Running or not it will suck air through the MVHR or from other rooms via the plenum. If you want MVHR to not contribute to fire air supply you need a fire dampers also.

Turning off a mechanical fan that is forcing air into rooms, for the sake of £100, is worth the extra effort imo.

 

All ground floor plaster-in vents have intumescent fire collars on them. 

[Spinny]

I think Halon not permitted for new installs now anyway. But I thought there were other gas options now, and used correctly they don't kill people.

 

What does the MVHR do to the smoke ? Extract it, or circulate it ?

 

 

[Nickfromwales]

58 minutes ago, Spinny said:

What does the MVHR do to the smoke ? Extract it, or circulate it ?

Nothing whatsoever.

 

I am only considering an option for not mechanically, purposefully pumping fresh air into a room (ablaze) to promote combustion. 

 

In a room / compartment that has the doors and windows shut, a fire becomes involved far less quickly; the MVHR would compromise this delay by ‘fanning the flames’.

 

PS, there are zero fire extinguishing or retarding gasses that wouldn’t kill a person in minutes, or less.

[-rick-]

On 28/06/2026 at 22:31, MikeSharp01 said:

The 'interior designer' has persuaded me to locate the isolators for the ovens on the kitchen island so they do not break up the aesthetic of the oven wall. These would be 1.4m from the Ovens across the gangway from the wall with the ovens. Each oven has a max draw of 3600W so 7.2kW with both on full power. Each of the two ovens will have there own 32A isolator in the Consumer Unit (CU), To achieve the run it ends up as a lot of 6mm cable: CU - Island (12m) & Island - Oven (15m) I am going to have 27m run (Voltage drop 3.5V approx), none in insulation but in duct and trunking. [I would need only 5m of 6mm cable if the switches were local to the ovens] So for these two ovens I will need just over 50m of cable which in itself is not a problem but I thought I would get a sanity check here to be sure I am not breaking any rules doing this - it looks OK to me regs wise.

 

Well this thread drifted. I'm just catching up after being away and nobody appears to have mentioned that each 3600W oven likely requires 16A protection in the CU not 32A. Check the manual. Normally the CU protection is there for just the cable, but for wired in appliances I think it also needs to be sized to protect the appliance. (TBC I'm not an electrician so check this with someone who is).

[Nickfromwales]

12 hours ago, -rick- said:

 

Well this thread drifted. I'm just catching up after being away and nobody appears to have mentioned that each 3600W oven likely requires 16A protection in the CU not 32A. Check the manual. Normally the CU protection is there for just the cable, but for wired in appliances I think it also needs to be sized to protect the appliance. (TBC I'm not an electrician so check this with someone who is).

This is standard detailing, for any spark. Was going though this today for 1ph ovens (x2), 3ph induction hob, 3ph ASHP's (x2), and 3ph immersion. Lots of very comprehensive discussion was had, and that means the CU will be sized, populated, and comply fully without any hiccups  

 

Breakers (or fuses) have always been sized historically, like 15a rewireable fuses for immersions, so this is not anything that's ever overlooked in my experience.