On Sat, 15 Dec 2018 15:44:12 -0000, trader_4 wrote:

On Saturday, December 15, 2018 at 9:50:53 AM UTC-5, Kristy Ogilvie wrote:
I'm in the process of fitting a Triton electric shower, and was being nosy. I noticed it has a thermal cutout situated where the water leaves the heater unit, this: http://www.datasheet-pdf.info/entry/36TXE11

That cuts out at 105C (ouch!) and it's also switching 20 amps when it's rated at 10 amps, AND it only cuts off ONE of the two elements.

It does have another cutout on the other end of the heater unit, which cuts them both off.

1) Why have two cutouts?
2) Why overload the first one?
3) Why only cut off half the power?

I think I have part of your answer. If you pull up the actual datasheet,
it says this:

36Ts (automatic reset) are supplied to customer specified open and close calibration set points with a tolerance on both set points.

Ah, I didn't read that far down.

It has a table showing the various versions and the temp ranges, tolerance
they can be *set* to. So, I think what you have is a part that is marked
as the generic, ie 105C cut-off, but the cut-in and cut-off can be set
by the manufacturer to the customer's reqts. What you have could certainly
be set to a lower cut-off temp.

That makes sense.

But having said that, the range of 71C
to 105C still sounds very high for a shower water heater. Any chance you
have the exact part number wrong? One letter or number difference matters.

Well presumably that's what it CAN be set to, and like you said, they haven't.

If you have the current figured out correctly and it's really 20A, then
this part should not be in there.

An estimate, but I knew it was more than the 10A I thought the stat was rated at. The shower is 8.5kW (35.5A at 240V), and I measured the resistances of the elements (when cold) as 11.3 and 14.75 ohms, with the stat switching the 11.3, which would be more than half the full load, i.e. more than 17.75A. I see further down the datasheet that the stat's actually rated 10A for 100,000 cycles and 16A for 30,000 cycles. Since it probably only kicks in if you overheat the shower, 30,000 cycles is ok. Maybe it's wired up wrong and it should be switching the smaller element? Or at 20A it can handle maybe 15,000 cycles?

As to having two cut-offs, that's very common. Typically there is a thermostat
for normal operation,

I don't think it's normal operation for the stat to operate - this is a basic shower where the element should be on all the time (1 or both depending on the user setting), the fine temperature control is by a water pressure control. So I assume if you overheat it slightly, then in warm mode the heater goes off, and in hot mode it cuts to half power, then if it overheats further, the other stat cuts both off.

then one or two thermal fuses that blow if it exceeds
normal operating range, eg it's being run with no water.

They're both stats as far as I can see, which auto-reset when it cools back down. It would be damn annoying if you had to replace one every time you overheated it.

If there are
two elements, it's also common in water heating applications for there to
be two thermostats to regulate the two elements, at least in tank type
heaters.