Hello, its been a while!

I have a heat pump with a 6kW 3ph star electric accessory heater

I thought it wasn't working properly, and to cut a long story short, I clamped the terminals, and got a load reading on only one phase.

I have drawn it out, to try and make sense of it. I have a theory as to why its working as it is, but ill get to that in a later post.

The only way I can see those readings occurring is if heater 1 is open circuit but with an earth fault at the right hand end. Heater 3 must also be o/c or have no supply.

The only way that those amperage readings make any sense to me is if HEAT3 (or LS3) is open and HEAT1 is shorted to ground on the line side.
That would leave HEAT2 as the only working element; but it's using the metal housing as the return path instead of being routed to L1 via the HEAT1 coil.

that's what I think is happening too. (its what I alluded to in the first post)

I don't think it would operate any cpd either. I didn't get a shock touching the case of the unit. It's still a safety issue though.

woulnt this happen if any element went down to ground though, and not operate the cpd? The only way to protect it would be to install a RCBO/RCD (GFI)

The breaker won't trip until it sees more current than it's rated at. Depending on where the short is the current value on any given phase may not get high enough.

A GFI might do the trick; but only if the return current isn't somehow coming back on one of the phase legs.
If this was something expensive I'd suggest a protective relay that would operate if the current on the 3 legs differ from one another.

its not something expensive, its just a heat pump with the accessory heater.

I can understand how it's all working as it is, it just sounds a bit alarming

it must be a fairly common failure mode for these things

My guess is it started with the ground fault on heater 1, that one blew along with #3 because now they are seeing 120v instead of 104v and #2 is living on borrowed time.

What is the name plate power consumption? 3KW?

6.9kW, and its 400v between phases, so if the earth fault is where you have drawn it, heater 2 is seeing 230v, rather than 400

The heaters are in series so the phase to phase voltage is split between them. Unfortunately I am still having the numbers to come out right with this calculator.
https://www.watlow.com/reference/tools/3phase.cfm

Greg,

Since there is no contactor for the third phase, they may be using a corner grounded delta. In that case, the heating element will see the full 400 V across it and the current will be approximately double of the calculated. That comes a bit closer to the measured values.

What are the voltages at thes points? Also. Humor me for a moment. Which heat elements are heating?

Greg, I think you may have your ground fault at the wrong end of the element

Even with a corner ground, I would imagine the voyages will be the same between the legs. By the readings, the heater lost the top contact likely to a ground fault between top contact ant first element and lost LS3 or bad connection on the jumper betwen LS1 and LS3. A compound problem

True. However the voltage between each phase to ground would be different. If the return path is ground instead of the other phases, the one working heater would see 400V across it instead of the expected 230V.

A floating 400V delta would see about 230V from each phase to earth. Ground one corner and now two of the phases would measure 400V to earth and the grounded phase would read 0V to earth.

I just realized we are all going down the wrong path.

6.9 KW total heater output = 2.3 KW per phase

2.3 KW per phase / 230 V phase to neutral / earth = 10 A

Measured current ~ 10 A.

Element resistance when hot, is around 23 ohms.

Top element is open, middle element is fine, bottom element is either opened or unpowered. Somewhere along the common connections is grounded. Probably at the top heating element, near the common connection point.

Agree! Without the calculations, I think that's what I suggested at first.
Whatever, I think you can safely say you need some new heater elements.

this is the UK, no corner grounded delta here!

its plain old 400/230. No neutral even to the isolator on this either

Ill get my multimeter out on Tuesday when I'm back

this is exactly what I think is happening. I think the top element is shorted to ground, but on the left hand side, but I cant explain why!


either way, its definitely fubar, just need to find somewhere to replace the elements on it

I disagree with the way that the diagram by OP was originally drawn....
For a start, it would be pretty poor form for an equipment supplier to wire a 3 phase 400V heating circuit in this manner, irregardless of the load or voltage.
What I mean by this, is the fact that under any electrical regulations, this configuration would not fly.
In any country that uses 230/400V, you need to feed your contactor coils with no more than 24VAC and make sure that the wiring through them is "straight through", as in no extra contacts outside of your main contactor.

This is obviously an extra heat circuit, when the weather gets really cold, with it being near 7kW, it seems to me like the contractor low-balled on the heat capacity of the original unit?

its drawn as the manufacturer manufactured it, its an assembled unit that just wires straight into the L1L2L2 terminals, the contactor (although the manufacturer calls it a relay) is 24v from the heat pump control circuit.

Its a 15kW heat pump, and this is just the accessory heater for it, designed for the unit, and fits inside it.

Trumpy,

I suspect the switches drawn as LS1, LS2, and LS3 are actually over temperature switches. I would expect to see them on the line side of the heating elements instead of the common. I am also surprised the contactor is not a three pole device. The lack of the third contact enables the bottom heater to be always on if the common gets grounded.

Assuming the system got approved by the relevant authorities, we can only offer opinions on a safer design.

Here are the manufacturers install instructions http://dms.hvacpartners.com/docs/1009/Public/01/AR1000001A.pdf

lS1, etc are over temperature switches.

I am surprised at the apparently poor design

Says you.... Lol!

If the LS switches are "limit switches" or over loads, I stick with my last post. Based on the amp loads, L1 grounded out between relay and HEAT 1, LS 3 is tripped or defective. LS 1 and 2 are still closed possibly welded together. As for the relay, may be defective with L1 is open and L2 of the relay is welded. That would explain the goofy readings and why the breaker did not trip

the ground fault, L1 cant be grounded, but the heater element must be grouded.

im saying that because if L1 was grounded out, the dead short would trip the breaker.


so, the relay is closed, heater 3, either the limit is stuck open or the heater is o/c. Heat 2 is fine, and heat 1, the element is grounded out, and not connected to L1.


We will find out tomorrow.


on a separate note, do the elements on these things go o/c often?

I think I might have a spare, so it might just get swapped out, but either way, ill find exactly whats happening tomorrow.

I have the unit on my desk now, one element is o/c, one is good and the other is in about five pieces. the ground fault is gone, probably during the move

the limit switches are all good