Motor Protection and Switching devices Co-ordination

Motor Protection and Switching devices Co-ordination

Three phase induction motors are the driving force behind all industrial processes. Automation is making motor control more sophisticated than ever before. However, motor protection co-ordination is an aspect which seems to be sidelined. While major industries and plants have been focusing on it, it seems to be neglected in smaller industries. We shall discuss here the need as well as benefits of perfect motor protection co-ordination(referred only as co-ordination).

A motor feeder typically consists of three devices as in the figure below :
1) Short Circuit Protection Device (SCPD , typically Fuse or MPCB).
2) Switching Device (Contactor)
3) Overload Protection Device (OLPD, electronic or thermal relay)

Co-ordination defines zones of protection for each device as well as its effects on the system. The typical tripping characteristic curve can be as below :

As you can see, in case of small overloads where the let through energy remains less than breaking capacity of Contactor, OLPD takes over the protection and trips the contactor. However, in case of higher fault levels, the SCPD trips (or blows in case of fuse) to ensure fast tripping and minimum let through energy.

The prevalent standard for switchgear IEC 60947 specifies types of co-ordination as below:
1)      Type 1 Co-ordination: No danger may be caused to persons on neighboring parts of the installation. The destruction of the contactor and the overload relay is permissible. If necessary, the contactor or Overload relay must be replaced. In case of short-circuit; the equipment need not be suitable for further services.
This is type of co-ordination is simple to achieve. However, every major fault shall entail replacement of components which is unviable for process industries. Hence, type 1 is not recommended in Industries.

2)      Type 2 Co-ordination : No danger may be caused to persons on neighboring parts of the installation. The overload relay may not suffer any damage. Slight welding of contacts is permissible which must be easily separable.  In case of short-circuit; the equipment should  be suitable for further services
This type of co-ordination is difficult to achieve and should be referred to manufacturers. As the system remains suitable for operation even in case of short circuit, such a co-ordination is recommended for process Industries.

Type 2 Co-ordination can be achieved by using either Fuse or MPCB as SCPD . For perfect co-ordination, all devices in the feeder should be from a single manufacturer.

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