Faulty 3 Phase Load

If one heater element short-circuits, the voltage in the other two will multiply by √3, the current by √3, and the wattage by 3. Possible burnout.

If one element fails open, the voltage in the other two will reduce by (√3)/2, the current by (√3)/2, and the wattage will be 3/4 of before. They won't get as hot.

I don't think a corner ground-fault will make any difference to the heater elements.

Thats reasonable i believe you but i can't stand by the concept unless i find a proof for it myself. Thank you sir.

The heater system is star connected, if one element goes open circuit the only problem you would have is that you would have an unballanced load as one phase would not be operational, it would also take longer to reach the target setpoint temperature, that is if it is able to reach the target temperature.

You have to understand that when the elements are star connected, and you have a resistive load connected ohm's law comes into play. Between phases you always have 2 elements connected in serries, if an element burns out and is open circuit you only have now 2 elements connected across 2 of the phases and this is exactly what you would have even when all the elements are operating.

Lets say the elements are 50 ohms each, when wired in serries that is 100 ohms across 2 phases, so V/R=I 400/100 = 4 amps this is the line current, this would not change as the load is resistive, all that would happen is you would is you would loose the heating ability of one of the elements, nothing else, there is no danger of having a cascading effect of burning out the other elements.

If you or your electrician are so concerned about this, you could always instal a 3 phase thermal overload to monitor any irregular phase imballance and raise an alarm or shut down the heater, which ever is perferred.

Also you mentioned about a voltage rise, the only voltage rise that could happen is to the phase that is not operating when the element is open circuit, but this voltage could not rise any higher than the supply voltage, as the circuit is not inductive.

I hope this answers your question,

Cheers

Joe

I dont think the thermal overload relay works here.In the case of open circuit fault in one phase of 3 phase load the voltage and current is reduced to sqrt3/2 and power to 3 in other 2 phases. It will work for short circuit fault. I was looking for the vector representation of faults in any phase. The answer is well explained with the help of triangle but i am so dump that i need more attention on topic. Thank you guys

You might not have practiced this much yet (or be out of practice), but you're asking the right questions and grasping the ideas.

I need to add a technicality to the earlier post 3. The triangle is an oriented triangle; think if it as having arrows going clockwise around. Although the corners are 60° angles, as vectors each side is rotated 120° from the next.

A higher level of generality about this is transformer vector groups. Wikipedia has a good article on this.

I would have to agree with you. You dont have much understanding of electrical control do you?

Let me explain: If you have a 3 phase overload in the circuit it monitors the circuit just as if an inductive motor was connected, if you loose an element it is the same as loosing a phase, the overload will trip because of the imballance of the phase currents.

Forget about all the other crap (no offence to other threads). Your question was about if an element went open circuit, my answer still stands.

Do you know what happens if you loose 2 Elements? You will have no heat, if they are wired in a floating star formation.

If you need to monitor the circuit, for an open circuit, use a 3 phase overload.

Trust me on this one.

Cheers

Joe

Overload devices don't detect phase imbalances or losses unless the remaining phase currents increase. This may happen with inductive loads like motors, but an open heater element in this scenario results in a modest drop of currents in the remaining elements, which goes right under the radar screen or normal overload devices. A phase protection relay (different animal) could work, though.

Tornado

Well I thought 3 phase overloads were about monitoring the current on each phase, I may be wrong, But tell me why when we are using a single phase load through a 3 phase overload we tend to wire the overload in series, modern overloads don't just monitor overcurrent, they also monitor the current in each phase, older thermal overloads also do the same thing. Try connecting a single phase load to a 3 phase overload, Without wiring the overload in serries.

Thermal Overloads on a 3 phase system heat at the same temperature, If we loose a phase on a motor, one of the overloads cools or possibly another overheats and this causes the tripping, Correct me if i am wrong.

Best Regards

Joe

The Truth Shall Set You Free

Joe, You are correct on all the points! GA

The OP must understand something about the flow of electric current and the laws (Ohms law at least...).

One element ==> open circuit ==> heat reduced by 1/3 from original setting with 3 on a 3 phase START. No changes in voltages on each phase. And yes, the normal 3 phase Overload used for inductive motors, with thermal bimetals ... will trip due to unbalanced heating of the elements.

Thanks for the GA

I Think I'll have to agree with Tornado.

On the old solder pot overloads a lost phase on a resistive load will not trip the overload since the 2 remaining phases do not exceed thier original current.

On newer style overloads they would likely trip on imballance, and not overload. For the same reason.

Elroy, you are correct about the solder pot overloads, these would not be suitable for this application, for the reason you stated.

When I mentioned older thermal overloads, I was refering to the Bimetal type of overload.

I have not seen a solder pot overload, for at least 20 years, in fact the last one I worked on was in 1988 in a factory in Dunedin, and I bet that factory is still using them.

Cheers

Joe

Elroy

thanks for introducing differential protection in 3-phase i don't know the concept is used for bimetallic strip for phase imbalance(apart from OL protection ).

Hi Elroy,

That was exactly what I meant to say, so you get a GA from me.

Your wording and detailed explanition was very good

Cheers

Joe

well guys

I'm interested to share some pics of the "oil filtering plant".It remind me of an old Russian military device in video games.

& the connection

....

It looks like you've got your work cut out for you. Safety must not have been a big concern where this machine came from. I'm guessing the wires in the bottom picture are connected to the heating elements but I don't see any filters.

Yes you are right.The star connected heaters draws 30Amps and i could see its connected directly with 30Amps capacity contact.There is only one OL relay for vacuum pump. The filter is in the front side enclosed. A lot of valves,termostat,pressure guage etc. its oil flow is alien for me.