I really have an interest in power quality and this forum has been very helpfull. Thank you all. The other day one of our boiler techs asked me to check the power going to there boiler. There flame signal voltage was all over place. I first checked the voltage and and between a-b and b-c was about 204 but between a-c was 199. Is this ok? If so why and if not why? What usually causes a voltage unballance.
Power quality
- Thread starter JdoubleU
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cadpoint
Senior Member
- Location
- Durham, NC
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Can we assume that the circuit is 208 ?
I'll bet that the high temperature wire insulation is breaking down.
But since it's a furnace I guess that it's only fired during a long work day and that heating and cooling have also worked on the connections and the wires.
Since it's been around since who knows when it's been reworked internally and they creted a choke point on the circuit with the problems.
I don't know what type of wire your dealing with but I'll assume it's about 115-C and just needs to be replaced.
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Can we assume that the circuit is 208 ?
I'll bet that the high temperature wire insulation is breaking down.
But since it's a furnace I guess that it's only fired during a long work day and that heating and cooling have also worked on the connections and the wires.
Since it's been around since who knows when it's been reworked internally and they creted a choke point on the circuit with the problems.
I don't know what type of wire your dealing with but I'll assume it's about 115-C and just needs to be replaced.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I would take this as a 208 volt system?
Is the gas control system 120 volts?
If not make sure any power requirements for the gas control system are rated to operate on 208, other wise look into a small transformer to step up from 208 to 240 volts, most flame rod type gas safety systems are electronic, and will have a power supply that has voltage regulation which will take quite a voltage drop before it goes out of regulation.
Also ask him to make sure the controls have the proper connection to the frame of the burner as this is part of the flame rod current path back to the control, I had a case one time where the tech tried to test a new control with out mounting it, and had the same problem with the flame rod current and voltage measurements, he didn't realize the mounting screws provided the rest of the path for the flame rod return.
If this is a 240 volt system and your getting those kind of voltage readings you definitely have a problem, but 199 volts on a 208 system is only a 4% voltage drop and should not be a problem.
As far as the imbalance goes, a heavy single phase load can cause this, or a loose connection, but at 4% it doesn't sound like much of a problem, I'm thinking wrong voltage supply for the gas control I.E. 208 instead of 240 volts, lost return path for flame rod (control or its transformer not bonded to chassis) or control not mounted.
Is the gas control system 120 volts?
If not make sure any power requirements for the gas control system are rated to operate on 208, other wise look into a small transformer to step up from 208 to 240 volts, most flame rod type gas safety systems are electronic, and will have a power supply that has voltage regulation which will take quite a voltage drop before it goes out of regulation.
Also ask him to make sure the controls have the proper connection to the frame of the burner as this is part of the flame rod current path back to the control, I had a case one time where the tech tried to test a new control with out mounting it, and had the same problem with the flame rod current and voltage measurements, he didn't realize the mounting screws provided the rest of the path for the flame rod return.
If this is a 240 volt system and your getting those kind of voltage readings you definitely have a problem, but 199 volts on a 208 system is only a 4% voltage drop and should not be a problem.
As far as the imbalance goes, a heavy single phase load can cause this, or a loose connection, but at 4% it doesn't sound like much of a problem, I'm thinking wrong voltage supply for the gas control I.E. 208 instead of 240 volts, lost return path for flame rod (control or its transformer not bonded to chassis) or control not mounted.
Last edited:
robbietan
Senior Member
- Location
- Antipolo City
In power quality, voltages of the supply and the equipment must be compatible. Assuming a 208 volt circuit, the utilization voltage of the furnace should also be 208 volts. If that is the case....
We go to the question, "Is 204V and 199V OK?" Again, we look at the equipment and the supply. If the equipment does not say what the voltage range it can operate in, we can assume it runs on plus/minus 10% of its utilization voltage (which is 208). Same is true with the utility supply, it can vary within a plus/minus 10% limit. Assuming it is a 208V system, the voltages between 228.8V to 187V are OK. that brings us to the next question....
Voltage unbalance - this is tricky, as there are several formulas for unbalance out there. NEMA says it is the maximum deviation from the average voltage divided by the average voltage. In your example, the unbalance 1.64% {(202.3-199)/202.3V} which is well within the economical operating liits of equipment. NEMA gives a max limit of 3% for voltage unbalance. Uneven distribution of single phase loads within a 3 phase system is one of the causes of voltage unbalance. Loose terminals is another
Hope this helps
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