Oven Replacement - RCD?

[lozarus]

Afternoon all. Apologies if this is a stupid question. I'm currently half part way through completing 2391 (second exam tonight) - so may be getting myself in a tiz overthinking things.

Either way, thought I'd put it out to the group.

Task is: Straight swap of existing x5 ovens with new - "like for like", if you will. Albeit cooker plate to appliance is also in T&E and I intend to replace this with a more suitable product - namely HO7.

Earthing arrangement is: TN-S.

Environment is: Educational establishment, cookery rooms.

Circuits for the room originate in a local 3P+E MEM2 board, no RCD's. Ovens are simply domestic 1P+E bog standard, cheap household jobs, wired from C32's at the DB to 45a Isolators (no socket) at each station, cooker outlet below. Room is only about 10m long.

EICR from 2020 is available, but quite a few tests missed out - IE: Zs for each oven measured well within limit. No IR or R1+R2 undertaken - I'll be doing this anyway. It's all PVC T&E - mainly in metal trunking. I think it's loose behind the cabinets for the last few metres however.

My big question here is RCD. I could take the "like for like" approach and just straight swap the ovens out as "maintenance". But the environment (education) makes me think I really should be pushing for RCD's.

Further to this, manufacturers instructions state "Additional protection by RCD is recommended".

With that in mind, I'm convincing myself these really should be going onto RCD shouldn't they?

Or convince me I'm wrong. Sorry if I've overcomplicated a simple job here.

Thanks.

 

[default]

I can't think of one that I've seen that was tripping breaker/fuse.
I guess it depends on where the element fails and if that element touches the casing.

So thinking above oven elements….and I’m definitely over thinking here….
An rcd tripping is fairly common if an element is on its way out.
If the failure mode is leakage to earth, is it fair to assume that if there was no RCD this could degrade over time, and the fault resistance could get low enough to make the cooker a bit tingly to the touch but not super-low enough to trip an over current device.
Or does it tend to end up open circuit before that happens?
I’m just wondering how this played out before we had RCDs.

When an element fails it is a dead short. Usually all three are down. Doesn't matter if its an emersion kettles, oven or any other element when they finally go it'd a dead short and the MCB,fuse,rcbo will trip. Most elements under 18 mega ohm will start tripping the protective device. I know some people will question that and yes your right it depends on the wattage of the element but in most domestic and commercial situations this is the fact.

 

[default]

When an element fails it is a dead short. Usually all three are down. Doesn't matter if its an emersion kettles, oven or any other element when they finally go it'd a dead short and the MCB,fuse,rcbo will trip. Most elements under 18 mega ohm will start tripping the protective device. I know some people will question that and yes your right it depends on the wattage of the element but in most domestic and commercial situations this is the fact.

And if the fault is between L&N then the RCD shouldn't be tripping first.

 

[default]

And if the fault is between L&N then the RCD shouldn't be tripping first.

Or more commonly

 

[mainline]

When an element fails it is a dead short. Usually all three are down. Doesn't matter if its an emersion kettles, oven or any other element

I have never seen a wire wound fire element go short, in fact, I don't think it's even possible as the heater element goes open in a failer and being as there is no earthed casing to contact it just sits there not working.

Most elements under 18 mega ohm will start tripping the protective device.

An 18 mega ohm element will trip ?????

That's around 0.0000127777778 amps, or 0.00293888889 watts.

and yes your right it depends on the wattage of the element but in most domestic and commercial situations this is the fact.

Nobody mentioned wattage.

When an element fails it will usually "open" the electrical circuit and no further heating will take place. When this happens, the element simply appears to have stopped working.In some rare instances the element will "short-out" against the sheath which is the outer visible part of the surface unit.

 

[default]

I have never seen a wire wound fire element go short, in fact, I don't think it's even possible as the heater element goes open in a failer and being as there is no earthed casing to contact it just sits there not working.

An 18 mega ohm element will trip ?????

That's around 0.0000127777778 amps, or 0.00293888889 watts.





Nobody mentioned wattage.

When an element fails it will usually "open" the electrical circuit and no further heating will take place. When this happens, the element simply appears to have stopped working.In some rare instances the element will "short-out" against the sheath which is the outer visible part of the surface unit.

I've been to numerous faults where the cooker circuit has tripped and will not reset. And the issue has always been the element. Same as 3kW emersion. Yes 18mw starts to nuisance trip. Don't know why this happens but it seems to.

 

[default]

I've been to numerous faults where the cooker circuit has tripped and will not reset. And the issue has always been the element. Same as 3kW emersion. Yes 18mw starts to nuisance trip. Don't know why this happens but it seems to.

If you can explain this then great.

 

[timhoward]

Let's keep this discussion civil gents!
I've certainly known elements that are measuring respectable values when cold but will trip an RCD when energised after a minute or two. It's been explained to me previously that this is due to trapped moisture in the element working its way to the ends.
I've also known elements fail open circuit. I believe I've seen short circuit but don't recall one

The reason I asked the question is that it's far more common for a customer to say "my RCD trips" if I turn the oven on than "my MCB trips". The early warning is commonly the RCD tripping.
That got me wondering if before the days of RCDs the leakage to earth could become noticeable to the touch.

 

[mainline]

If you can explain this then great.

I can explain why 3 ma of current wouldn't cause a trip, but not the other way around.

 

[mainline]

Let's keep this discussion civil gents!
I've certainly known elements that are measuring respectable values when cold but will trip an RCD when energised after a minute or two. It's been explained to me previously that this is due to trapped moisture in the element working its way to the ends.
I've also known elements fail open circuit. I believe I've seen short circuit but don't recall one

The reason I asked the question is that it's far more common for a customer to say "my RCD trips" if I turn the oven on than "my MCB trips". The early warning is commonly the RCD tripping.
That got me wondering if before the days of RCDs the leakage to earth could become noticeable to the touch.

It's also caused by the element expanding and contracting, which then causes the filler to break down.

 

[mainline]

Let's keep this discussion civil gents!
I've certainly known elements that are measuring respectable values when cold but will trip an RCD when energised after a minute or two. It's been explained to me previously that this is due to trapped moisture in the element working its way to the ends.
I've also known elements fail open circuit. I believe I've seen short circuit but don't recall one

The reason I asked the question is that it's far more common for a customer to say "my RCD trips" if I turn the oven on than "my MCB trips". The early warning is commonly the RCD tripping.
That got me wondering if before the days of RCDs the leakage to earth could become noticeable to the touch.

I guess so, especially on a TT earthing system that is going to have a high earth loop impedance, but personally have never come across such an occurrence.

 

[brianmoooore]

If you're quick enough with the megger after the RCD trips, you will see a rapidly increasing resistance. The resistance when measured cold is orders of magnitude higher than when the hot element trips the RCD. Presumably caused by the element moving by expansion/contraction.
On the type of elements we are discussing, the live and neutral terminals have a foot or so of element between them, so L - N shorts aren't normally possible.
I was around long before RCDs, and it wasn't at all unusual to see holes blown in the outer sheath of these elements, or even severed completely.