ECN Forum Forums General Topics of the Trade General Discussion Area 3 phase star heater confusion

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.