480 3-phase heater

[Busarider29]

If I have a 3-phase heater (480 VAC) where its controlled via a contactor, if one of the contacts of the contactor burns shut, will it pop the fuse on one of the other two legs of the heater (all 3 legs are fused)? Using either delta or Wye heater? I don't think it would and I believe that the heating element on the leg where the contact is burned shut will just heat as normal, where the other two won't, but none of the fuses will blow. Confirmation please

 

[texie]

Assuming you have a 3 pole contactor and the contactor is deenergized, if one pole welds closed the heat will not draw any current. There is no return path.

 

[Busarider29]

Assuming you have a 3 pole contactor and the contactor is deenergized, if one pole welds closed the heat will not draw any current. There is no return path.

Ok, that makes sense. My colleague is suggesting that we use an overload like this on the bottom of the contactor. He thinks that if one of the contacts welds shut, then the heater will draw more current and pop the overload. I believe these overloads are for 3-phase motors, and will not work for a 3-phase heaters like he is thinking.

 

[Busarider29]

As far as no return path, what about something like this? Isn't the N the return path? If one contact welds shut while the others are open, will it result in a blown fuse and/or will the leg with the welded contact draw more current if the other two contacts are open?

 

[texie]

You indicated the heater is 480 not 480/277 so there should not be a neutral involved.

 

[Busarider29]

You indicated the heater is 480 not 480/277 so there should not be a neutral involved.

Ok

 

[Jraef]

You indicated the heater is 480 not 480/277 so there should not be a neutral involved.

Unless of course, someone connected it wrong and ran the neutral because they saw the diagram of the elements connected in Wye and made an ASSumption. I’ve seen that.

However, if there was a neutral connected to the Wye point and a single contact welded, that would have no effect on the other two circuits. You would just draw normal current on that one leg. So it can’t be that from the description of the symptoms.

One contact welded on a heater generally means the heater heater coil is failing, often by arcing to ground. That draws high current, but because it’s a high resistance fault, not high enough to clear the fuse. That is, until you add the normal arcing that takes place when you open one of the other two poles of the contactor. The two arcs in series drive the voltage higher than normal which, in a resistive circuit, increases the current to where it does clear a fuse. Which one clears first is just coincidental to which phase angle was highest at the moment the contactor opened.

 

[Besoeker3]

Assuming you have a 3 pole contactor and the contactor is deenergized, if one pole welds closed the heat will not draw any current. There is no return path.

Agreed for delta but won't there be if it is wye connected?

 

[bwat]

Agreed for delta but won't there be if it is wye connected?

Depends on if the neutral was actually run or not as Jraef pointed out. Wye connection doesn’t require a neutral conductor. Similar with motor connections how it can be delta or wye, depending on voltage, etc. But it doesn’t mean you bring the neutral.

 

[kwired]

My experience has always been if one contact welds shut, contactor can not mechanically drop out and all three poles remain closed and the load never stops.

But if you do have a situation where only one circuit conductor is connected to the source, including a neutral if used, there is no return path and no current will flow.

If you do have two of three lines current will remain same as it normally is if the heater is delta connected, current will actually drop a little if they are wye connected with a floating wye point because the voltage across individual elements will go from 277 to 240 in that case.

 

[Besoeker3]

Depends on if the neutral was actually run or not as Jraef pointed out. Wye connection doesn’t require a neutral conductor. Similar with motor connections how it can be delta or wye, depending on voltage, etc. But it doesn’t mean you bring the neutral.

Fair point. I assumed that, if it was wye connected, that the star point would actually be connected. Configured as a wye might have removed that possible ambiguity. No big deal.

 

[Russs57]

"If I have a 3-phase heater (480 VAC) where its controlled via a contactor, if one of the contacts of the contactor burns shut, will it pop the fuse on one of the other two legs of the heater (all 3 legs are fused)? Using either delta or Wye heater? I don't think it would and I believe that the heating element on the leg where the contact is burned shut will just heat as normal, where the other two won't, but none of the fuses will blow. Confirmation please"

Sorry I am late to the party. The worst mess of burnt up contactors and blown fuses I have ever seen was on a 3 phase 480 water heater (reheat water FWIW).

Jraef offers sage advice with regards to high resistance faults via heater elements shorting to ground (and sometimes ground is through mineral deposits). I'll also point out that said elements can be arranged to operate in WYE in low heating mode and DELTA in high fire mode. It can get confusing and involve a lot of contactors and blown fuses that don't seem to correlate at first.

So yes, given a complicated enough system you can have fuses blowing that simply don't add up at first.

 

[German shepards]

I think if you have a 3 phase heater and one fuse blows yes the other ones will blow since it’s in a single phase condition and the other 2 phases will try and carry the current

 

[augie47]

I think if you have a 3 phase heater and one fuse blows yes the other ones will blow since it’s in a single phase condition and the other 2 phases will try and carry the current

I don't believe you will find that the case on a restive heater. The loss of one phase should not cause an increase in current in the other two.

 

[Eddie702]

i agree with kwired that if the contactor welds one contact then all 3 contacts will stay closed and you will have a run away (overheat) condition......assuming the heaters are not defective.

If one element is defective (open & not grounded) the other 2 legs will draw normal current and the heater will continue to operate at less than normal total output..........

 

[kwired]

I think if you have a 3 phase heater and one fuse blows yes the other ones will blow since it’s in a single phase condition and the other 2 phases will try and carry the current

Go study Ohm's and Kirchoff's laws.

This is a resistance type load, resistance will remain about the same (maybe somewhat negligible differences if there is a temp change happening here but otherwise can assume resistance will still be same). An inductive load like a motor will have different impedance characteristics if a change like lost phase is introduced. But for a resistance only load you just lose whatever portion of the circuit that is directly connected to the lost input.

Now it gets a little more complex with three phase, in particular if the heating elements are connected in a wye configuration and there is no source neutral connected to the wye point. I believe I mentioned this in an earlier reply but will do so again. In that situation presuming 480 volts supply volts, if you connected the heaters in a wye configuration they will see 277 volts across each heater presuming they are of all equal resistance and there is no source neutral connected to the wye point. Now if you open one of the three input leads that leaves you with only two equal resistances in series with one another and with 480 volts supplied. Because they are equal resistance half the supply volts will drop across each element - meaning they only see 240 volts in this condition instead of 277. that is actually going to result in a little bit of a drop in current across each remaining element compared to if all three inputs were there.

 

[Jraef]

My experience has always been if one contact welds shut, contactor can not mechanically drop out and all three poles remain closed and the load never stops. ...

Depends on the contactor. Those with what's called a "positive drive" armature will do as you describe; the moveable contacts are hard fixed to the armature, so if one of them welds, the entire armature stays in that position. Those with "floating contacts" will not; the armature returns to the normal position and any un-welded contacts still open, only the welded one remains closed. NEMA design contactors will have floating contacts. Cheap DP contactors (and now, "Safety Contactors) have positively driven contacts.

That said, many many heater control systems use cheap DP contactors, so practically speaking, if one contact welds, they all stay stuck.

 

[kwired]

Depends on the contactor. Those with what's called a "positive drive" armature will do as you describe; the moveable contacts are hard fixed to the armature, so if one of them welds, the entire armature stays in that position. Those with "floating contacts" will not; the armature returns to the normal position and any un-welded contacts still open, only the welded one remains closed. NEMA design contactors will have floating contacts. Cheap DP contactors (and now, "Safety Contactors) have positively driven contacts.

That said, many many heater control systems use cheap DP contactors, so practically speaking, if one contact welds, they all stay stuck.

I agree NEMA contactors more likely to hold in every pole when only one is welded. I know I have seen NEMA contactors welded and all poles closed, but may have had more than one pole that was actually welded. Haven't really ran into all that many NEMA contactors that were welded in the first place.

I have seen many DP contactors that are still a floating design, but apparently don't have enough float range and they still end up sticking with all poles closed. Maybe seen a few IEC contactors that done this also.