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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Oct 2018, 6:00 pm   #1
Andrew B
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Default Switch rating.

Good Evening everyone.

A non functioning "instant" water boiler came into my 'shop today. Basically it's a flash boiler, it has an on/off spst power switch and a spst "heat" switch. The fault was found to be the on/off switch gone o/c. The switch is only used to "enable" the heater and actually passes no current at switch on, the "switching being done by the large 10A heat switch, effectively the on/off switch is only used to enable the boiler.
The defective switch is rated at 6.5A 240v, putting my multimeter in leu of the switch, i find the boiler consumes 10.7A when it's doing it's job and while the "heat" switch is actually switching the current, the on/off switch is rarely used, being maintained in the "on" position.

Is this just a case of the "bean counters" at work using a 6.5A switch (that never switches) or does anyone think it is adequate considering it only passes current and never switches a load?
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Old 12th Oct 2018, 7:03 pm   #2
Boater Sam
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Default Re: Switch rating.

Nasty inductive load from the contactor? Needs a snubber network?
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Old 12th Oct 2018, 7:30 pm   #3
broadgage
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Default Re: Switch rating.

No contactor involved I suspect.

Two switches in series both of which carry the load current. The actual switching being done by a fully rated switch, whilst the lower rated switch carries the same current but does not normally switch it.

Possibly acceptable, the makers of the switch MIGHT give it a higher rating for infrequent switching of a resistive load.
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Old 12th Oct 2018, 7:49 pm   #4
John10b
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Default Re: Switch rating.

Hi Andrew hope you mind me asking but how in basic terms does an Instant Boiler work? I have a very simple understanding of a Flash Boiler for raising steam.
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Old 12th Oct 2018, 8:33 pm   #5
emeritus
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Default Re: Switch rating.

I guess the switching arrangement is akin to the main switch of a traditional fused consumer unit, where the switch might have a current-carrying rating of (say) 60A. It would only rarely be called upon to interrupt a current of this magnitude, and would probably not last long if it did so on a regular basis.
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Old 12th Oct 2018, 10:58 pm   #6
broadgage
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Default Re: Switch rating.

Quote:
Originally Posted by John10b View Post
Hi Andrew hope you mind me asking but how in basic terms does an Instant Boiler work? I have a very simple understanding of a Flash Boiler for raising steam.
Cheers
John
I suspect that this may be a domestic "instant" boiling water tap as heavily advertised on TV at present.
These consist of a very small insulated pressurised hot water tank holding perhaps a litre of boiling water, typically installed under a kitchen worktop.

The stored boiling water may be drawn off almost instantly to prepare hot drinks or instant soup etc.

Once the whole contents have been used, a delay of some minutes results whilst the relatively low loading element heats more.

Catering or industrial versions exist but these have much higher loadings, I have seen a 28Kw unit.
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Old 13th Oct 2018, 12:35 am   #7
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Default Re: Switch rating.

10A is not particularly low loading at 2.4kW.

Is a flash boiler one where the current passes transversely directly through the water passing through a narrow insulated channel with solidly grounded metal inlet and outlet connections? Adjusting the inlet flow rate will give anything from high flow lukewarm to low flow boiling water outlet. The outlet is always open, so no risk of explosion and no flow at all simply shuts off the (electric) current.
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Old 13th Oct 2018, 11:01 am   #8
Andrew B
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Default Re: Switch rating.

Thanks for the replies.
It works by having a 1 litre container of cold water which is pumped at a low rate through a heating tube (2.4KW) and out of the spout into your cup, it sort of dribbles out along with puffs of steam and couple of seconds after pushing the "heat" switch. I can imagine in a hard water area the whole thing would have a short life when the tube "furrs" up.

You are exactly right on the operation Chris.

Like most of these domestic "farkles" it's poorly designed and constructed but keeps the masses amused making eco-heated water for coffee while listening to music delivered from THAT >hi-fi< sound via oxygen-free cables.

I have not yet seen a well insulated kettle where, when the water is boiled the heat doesn't escape in minutes?
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Old 13th Oct 2018, 12:00 pm   #9
John10b
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Default Re: Switch rating.

I can understand why you describe it as a sort of Flash Boiler, thanks.
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