UL508 question

Maybe someone with UL experience can settle something for me. In a UL508A listed panel, do you have to interlock the coil of a self protected combination motor controller?

On NEMA style starters you always see the coil interlocked with the overload contact, so that in an overload condition, the contactor breaks the power conductors.

On the newer IEC style self protected combination starters, the overload/circuit breaker section physically breaks the power conductors in an overload/short circuit condition, so I wouldn't think the coil interlock is necessary, but I have seen it quite a bit.

I can't seem to find anything in UL508A (control panels) or UL508 (equipment) specs that definitevly says either way. What say you?

Maybe I'm confused but on the self protected combination starters isn't the coil interlock just built in? Or do you mean running another coil interlock?

IE:

The contactor can still be pulled in when the starter is tripped and the power conductors are broken, at least on the Eaton and Allen Bradley ones that I have used. Never used Siemens, but I imagine it is the same.

I think interlocking the coil when using a MPCB or similar is just a carryover from the old days. It really isn't necessary with the component grouping you are referring to.

I don't think UL508 would get so deep into specific circuit design that it would make a comment on the question you ask. In effect they would say that the motor must be disconnected from the source if an overload is detected and that recognized devices must be applied in accordance with manufacturers installation instructions. If you take those two points together you can use a whole slew of different protection schemes and still fall in line with a single directive. It becomes the protection scheme that must be evaluated to meet a specific UL standard, not the implementation.

Keith

The only mention I find of linking something is in UL508A 34.3.3 which basically says if your overload is a thermal device that is inside the motor, then is needs to have provisions to be connected to the ungrounded conductor of the motor controller coil circuit.

There is good practice and there is code. These are not always the same thing.

You do not need to interlock the coil on a self protected starter. UL could not care less, not their thing.

But you SHOULD interlock it in most cases. If not, the trip device could be open from a trip, but the contractor could still be energized. Makes no difference in protecting the motor or circuit of course, but it DOES introduce the possibility of the motor instantly running again as soon as the device is reset. It you were using 3 wire control, you have defeated the safety aspect of 3 wire control.