How much effect should the kettle have on the microwave?

[cjard]

I've long wondered: in the house I currently live in, if the microwave is on and one turns on the kettle, the microwave slows down / changes fan noise due to the extra load from the kettle. I've never really noticed this in any other house I've had, which makes me wonder about the kitchen ring final and whether it has a fault. Would measuring the voltage at the microwave socket while the kettle is active tell me anything?

 

[JohnW2]

I've long wondered: in the house I currently live in, if the microwave is on and one turns on the kettle, the microwave slows down / changes fan noise due to the extra load from the kettle. I've never really noticed this in any other house I've had, which makes me wonder about the kitchen ring final and whether it has a fault. Would measuring the voltage at the microwave socket while the kettle is active tell me anything?

If you can do it safely, measuring the voltage and seeing how much it changes when you turn the kettle on would certainly give some indication of what is going on.

If it is a standard 32A ring final, you would not expect to see more than about 13-14 volts, at most, volts drop with the kettle, but I wouldn't expect that to have a noticeable effect on the microwave. If the drop is appreciably greater than that, this would suggest a poor connection somewhere in the circuit, which would need to be investigated, since it could present a fire risk.

Kind Regards, John
Edit: Typo corrected

 

[John D v2.0]

Ours does that too, a bit more so if the kettle is in the ring than in the cooker socket. Never really worried about it, I've tested the ring and it does have a relatively low IR at one point but the conductors are all fine.
I assume it's partly the old 1940s or so incomer, some kind of paper insulated conductor with hardly any meat on it. I think PFC was only about 700A although I'd be interested to know what normal is.

 

[JohnW2]

Ours does that too, a bit more so if the kettle is in the ring than in the cooker socket. Never really worried about it, I've tested the ring and it does have a relatively low IR at one point but the conductors are all fine.

I mistyped the figures in my earlier post (and will correct them). If it's a standard 32A ring final (with a B32 MCB), then the maximum permitted Zs is 1.37Ω, which probably corresponds to a L-N loop impedance of not much over 1Ω. If the Zs was 'just compliant', one would therefore expect around a 13V voltage drop (at the kettle) as a result of applying a 3kW (kettle) load. If the kettle and microwave were at similar points on the ring, then the microwave would see a similar fall - although (given the range of voltages over which it was designed to work) I would not really have expected that to have a noticeable effect on the behaviour of the microwave. With a Zs appreciably below the 'maximum permitted', the voltage drop would obviously be less.

Kind Regards, John

 

[John D v2.0]

Even the hand dryers at IKEA, when the other one comes on the first one's note changes noticeably. The problem comes when the appliance fails to work satisfactorily, but unless it's a piano the tuning doesn't matter.
Theoretically you could make something that converts the voltage into a tune, and change to a different tune every for every whole volt

 

[AdrianUK]

I mistyped the figures in my earlier post (and will correct them). If it's a standard 32A ring final (with a B32 MCB), then the maximum permitted Zs is 1.37Ω, which probably corresponds to a L-N loop impedance of not much over 1Ω. If the Zs was 'just compliant', one would therefore expect around a 13V voltage drop (at the kettle) as a result of applying a 3kW (kettle) load. If the kettle and microwave were at similar points on the ring, then the microwave would see a similar fall - although (given the range of voltages over which it was designed to work) I would not really have expected that to have a noticeable effect on the behaviour of the microwave. With a Zs appreciably below the 'maximum permitted', the voltage drop would obviously be less.

Kind Regards, John

John,

I think you need to re-think the difference between the L-N impedance & Zs.

If the (acceptable) L-N impedance really was as high as 1R then the PSSC would be in the order of 230A for a L-N bolted short circuit. This magnitude of fault current is only just acceptable for a B32A MCB and would not be acceptable for a C32A or anything bigger to obtain a 5s disconnection time.

The value of the L-N impedance needs be significantly less than Zs for a workable supply, otherwise, as you have calculated, volt drops will be intolerable. 13V drop for a 3kW load is unacceptable. Consider the volt drop that would be experienced by a 10kW electric shower and what its true output would become if the supply impedance really was about 1R.

As food for thought.... consider the max Zs for a PME supply..... Zs, for a PME service, is, in effect, the L-N impedance. The max acceptable value for Zs on PME is 0.35R.

Adrian

 

[John D v2.0]

Adrian he was talking about the ln impedence at the end of a long circuit, indeed at the incomer it should be much better.
Your shower circuit would need better than 1 ohm because of the volt drop calculation, hence why a shower at the bottom of the garden would end up on 16mm cable but next to the cu 2.5mm would be enough assuming the insulation could cope with the heat!

 

[JohnW2]

John, I think you need to re-think the difference between the L-N impedance & Zs. If the (acceptable) L-N impedance really was as high as 1R then the PSSC would be in the order of 230A for a L-N bolted short circuit. This magnitude of fault current is only just acceptable for a B32A MCB and would not be acceptable for a C32A or anything bigger to obtain a 5s disconnection time.

I don't need to re-think anything - as I explain at the end of this post, I think you are talking at 'cross purposes' with what I've been saying. What you describe is exactly what I was talking about - a final circuit that only just had a low enough Zs to satisfy the requirements of a B32 (which is very probably what the OP's ring final is protected by).

The value of the L-N impedance needs be significantly less than Zs for a workable supply, otherwise, as you have calculated, volt drops will be intolerable. 13V drop for a 3kW load is unacceptable. ...

You may call it 'unacceptable' but L-N and L-E ('Zs') loop impedances will only differ by virtue of the difference between the impedances of the N and E paths back to the transformer. In a TN-C-S installation they will, by definition, be identical at the origin of the installation, and very similar when view from loads on final circuits which are close to the origin/CU.

As for 'not acceptable', a 13V VD (resulting from my arbitrarily chosen example figures) is only slightly more than the 'guidance' figure of 5% maximum suggested in the regs, and the VD within the installation would then be appreciably under 5%.

Consider the volt drop that would be experienced by a 10kW electric shower and what its true output would become if the supply impedance really was about 1R.

Both Zs and L-N impedance (at the shower) would obviously be much lower for a shower circuit than the ring final we were discussing. Again, the circuit would be designed so that the VD under load was acceptable (in terms of the 'guidance' 5% in the regs, or whatever)

As food for thought.... consider the max Zs for a PME supply..... Zs, for a PME service, is, in effect, the L-N impedance. The max acceptable value for Zs on PME is 0.35R.

Ah, I think I see your problem - you are seemingly thinking about the situation (loop impedances and voltage) at the origin of an installation, not at the end of a final circuit.

Yes, at the origin of a TN-C-S installation, the EFLI ('Zs' if you want) would be a maximum of 0.35Ω. However, there could well be, say, 1Ω in the final circuit wiring of a 32A circuit, so that the Zs of the circuit at the load could be 1.35Ω (still within requirements of a B32). Assuming 2.5mm² T+E cable, if the R1+R2 of the final circuit (at the load) were, per my example, 1Ω, then the L+N impedance ('R1+Rn') of the final circuit would be about 0.77Ω. The total L+N loop impedance at the load would therefore be about 1.12Ω (0.35Ω+0.77Ω), hence a voltage drop (at the load) of about 14.56V with a 13A load - even if you regarded that as 'unacceptable'.

Kind Regards, John

 

[endecotp]

The difference in frequency between one note and the next on a piano keyboard (including black and white keys) is a about 6%.
So if you have a motor or something whose pitch depends on the supply voltage, if the relationship is reasonably linear then a 5% change in voltage should be easy to hear, while a 1% change might be too subtle for most of us to notice.

 

[JohnW2]

The difference in frequency between one note and the next on a piano keyboard (including black and white keys) is a about 6%. So if you have a motor or something whose pitch depends on the supply voltage, if the relationship is reasonably linear then a 5% change in voltage should be easy to hear, while a 1% change might be too subtle for most of us to notice.

That seems consistent with what I've been saying. As I've said, in a Zs-marginal 32A circuit, a 3kW load would probably produce a voltage change in the ballpark of 5%, so it would not surprising if the change in speed of a fan whose speed was voltage-dependent was audibly apparent. However, whether the fan in the OP's microwave would behave like that (rather than being 'synchronous'), I don't know.

Kind Regards, John

 

[securespark]

Do they normally get on well?

 

[JohnW2]

Do they normally get on well?

Who?

Kind Regards, John

 

[securespark]

The kettle and the microwave, of course!

 

[JohnW2]

The kettle and the microwave, of course!

Ah, I see! I thought we'd been told that they did not "get on well"? The OP did not suggest that the phenomenon described was a new one.

Kind Regards, John