UL 508A differences between control transfomer and power transfomer

[dawsez]

i dont know if i can make this question here but what is the difference between control transfomer and power transfomer in the UL 508A standard, I ask there is different value to calculate the protection for each type.

I have a 200VA transfomer and I just have one safety interlock relay, thank you!

 

[Jraef]

There is no difference in terms of what you do for protection, no. Internally, CPTs are designed to handle high inrush currents without much voltage drop, that's why they are more expensive for the same VA value.

 

[petersonra]

The difference is in what you are powering. If the output is just powering control service it's a control Transformer. If there are power circuits like Motors the Transformer is powering it is a power transformer. As for protection there is no difference. There are some differences in how you deal with the short circuit current rating calculations between the two. As a practical matter I have never had a UL 508a panel that had a power transformer in it. If I'm going to have a power transformer it is not going to be inside the ul508a panel.

 

[drktmplr12]

straight out of square d catalog:

Type T transformers are designed with low impedance windings for excellent voltageregulation and can accommodate the high inrush current associated with contactors,starters, solenoids, and relays. Type T transformers are manufactured using the mostadvanced insulating materials and are the best choice if size and cost are of concern.

Type TF transformers include factory-installed primary and secondary fuse blocks. TypeTF transformers consist of two primary fuse blocks and one secondary fuse block. Theprimary includes rejection-style clips to increase the AIC ratings for the fuses. Since thefuse blocks are mounted on the top of the transformer, Type TF transformers areinterchangeable with Type T transformers except for their increased height.

from what i understand, control transformers are available in very low VA ratings (50 to 5000, depending on mfr.).

 

[petersonra]

Since he asked about UL508a, here is what the actual standard says.

2.12 CONTROL TRANSFORMER – A transformer whose secondary supplies power to control circuit
devices only (excluding loads).

2.42 POWER TRANSFORMER – A transformer whose secondary winding supplies power to loads or
a combination of loads and control circuit devices operating at the secondary voltage.

2.32 LOAD – A device external to the industrial control panel that is connected to the power circuit.