125 Vdc vs 120 Vac for local control stations
125 Vdc vs 120 Vac for local control stations
(OP)
Hi,
Anyone knows an standard or document where it states that in a plant is not a good practice to have some local control stations with 120 Vac and others with 125 Vdc?. At the project where I am, it was proposed to have Medium voltage motors with LCS´s control voltage of 125 Vdc and low voltage motors with LCS voltage of 120 Vac. To me it make sense to have only one voltage going from the MCC to the Motors local control panels, no matter if it is Medium or low voltage motor.
Your opinion is wellcome
Regards
Anyone knows an standard or document where it states that in a plant is not a good practice to have some local control stations with 120 Vac and others with 125 Vdc?. At the project where I am, it was proposed to have Medium voltage motors with LCS´s control voltage of 125 Vdc and low voltage motors with LCS voltage of 120 Vac. To me it make sense to have only one voltage going from the MCC to the Motors local control panels, no matter if it is Medium or low voltage motor.
Your opinion is wellcome
Regards





RE: 125 Vdc vs 120 Vac for local control stations
There are good reasons for using 125 V dc for circuit breaker control in critical applications.
RE: 125 Vdc vs 120 Vac for local control stations
At medium voltage, protection of the circuits is probably performed by circuit breakers that are tripped by protection relays using some form of auxiliary power for the trip coil. If AC is used, then in the event of a fault, supply voltage may dip too low to trip the circuit breaker. Therefore, DC is used.
Because of the above, medium voltage circuit breakers are much more readily available with DC trip coils, while MCC's are much more readily available with AC controls.
RE: 125 Vdc vs 120 Vac for local control stations
RE: 125 Vdc vs 120 Vac for local control stations
Regards,
Cmelguet
RE: 125 Vdc vs 120 Vac for local control stations
RE: 125 Vdc vs 120 Vac for local control stations
In the LV do you have LV Circuit Breakers? If yes are they electrically operated (charging / tripping closing)? If yes is it AC or universal (AC/DC).
It used to be that LV Switchgear had AC control as it was easier / less expensive to have a control power transformer to supply AC loads. This all works well until the bus is dead. In light of arc flash concerns, how is the breaker closed remotely / electrically if there is no source of AC control power?
I have been in a few plants that have 125DC, 48DC,24DC and 120AC all in the same room (+primary voltages). Each voltage being for a specific system (protection/control, fire alarms, DCS, 480V MCC, etc).
If the DC system is sized properly, there is no reason not to add it to LV, other than cost and additional equipment. You have to be careful that the LV equipment can accept the DC.