UL 508 A: Industrial Control Panels

UL 508A - Traditional Combination Starters in North America

Protections defined by UL 508A and different types of motor starters chosen according to the protection devices

In the US there are several types of motor starters (Type A, Type B, Type C, Type D, Type E and Type F) which differ according to the protection devices used.Type A motor starter, for example, includes a manual disconnect switch, a fuse, a motor controller and a overload relay. Type A is the only motor starter that requires a fuse, which plays the role of magnetic protection and therefore of Branch Circuit Protection Device (BCPD), i.e. the device supposed to guarantee protection against short circuit currents and delimits the Branch Circuit.

Type B, which is no longer used today, includes a manual disconnect switch, a pure magnetic motor short circuit protector, a motor controller and a overload relay.

Type C, on the other hand, differs from Type A and Type B because, instead of a fuse and a motor short circuit protector, it includes an inverse time circuit breaker that not only provides a magnetic and thermal protection but ensures also the same function of a disconnect switch.

Type D, which is not very used, has an instantaneous trip circuit breaker (that fulfils the functions of a disconnect switch and a short circuit protection), a motor controller and a  overload relay.

Type E, accepted by UL in 1990, was born as a manual self-protected combination motor controller (disconnect switch, overload relay and short circuit protection in the same device). Today, Type E motor starter includes also a motor controller, thus offering a compact device called self-protected combination motor controller consisting of a manual self-protected combination motor controller plus a motor controller.

Type F (accepted by UL only in 2002) means the well-known "European" motor starter, i.e. manual self-protected combination motor controller + separate motor controller. In this case the coordination between the motor controller and protection inside the manual self-protected combination motor controller plays a fundamental role in terms of SCCR.

The definitions adopted in UL 508A regarding the roles played by protections are:

OVERCURRENT PROTECTION: overload, short circuit and fault to ground protection.
OVERLOAD PROTECTION: protection required for motor circuits, whose purpose is to avoid excessive overheating due to overloads.
The term combination motor controller refers to the combination of devices to provide means of circuit disconnection, branch circuit protection (short-circuit), motor control and motor overload protection. Inside the device one can find:

Motor Starter: the combination of overheating protection and motor controller.
Manual Motor Controller: the combination of overload relaydisconnect switch and short circuit protection.

Difference with European Motor Starters

In North American motor starters, each motor controller designated to the corresponding motor is associated with a control circuit whose purpose is to control the closing and opening of that motor controller. This means that there is a control circuit for each engine start.
The solution adopted in Europe, on the other hand, includes a centralised system with a single PLC that controls the motor controllers of the various motor starters.

 

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