Pump Motor Protection Question

i assume that there is also a suitable contactor which is tripped by the relay. From what you say, on locked-rotor current (LRA in USA), the bimetal relay is not tripping, but the main incomer is tripping (is it an MCCB with Thermomagnetic trip?), This indicates to me that the bimetal relay is not set properly, or has an unacceptably high trip time at LRA. You need to check the relay characteristic. As per IEC 60947-4, there are specific gate limits for this characteristic, for each trip class - 10A, 10, 20 and 30. There are also co-ordination tests between relay and the SCPD (fuse/MCCB) given in the standards. If you are in USA, you may need to check with UL and NEMA standards.

In summary, to avoid nuisance tripping of the incomer, you must coordinate the characteristics of both correctly...

• for overloads within the capacity of the contactor, the relay must trip
• for overloads beyond the capacity of the contactor, the incomer must trip, and
• for high short circuits, fuse must blow.

Please also check this CR4 thread :http://cr4.globalspec.com/thread/21499/Type-2-Coordination-Chart-for-Electrical-Components

sir,

Bimetallic o/L trip class is 10. The module is Switch fuse unit. the incomer of bus is ACB with electric make CAG relays set at 800A. Their settin can not be increased due to upstream system. The relay characterstic have no problem. The only reason is the relay or fuses does not get enough time to operate.

rattanpk@yahoo.com

Hi, long time no see...

Good. Well, your starting time is too long obviously. You need to see why, and whether it can be reduced by starting on lesser load. Or use reduced voltage starting methods like ATS, Softstarer etc.

If none of this is viable for you, you have to set the CAG relay somewhat higher. If there is no setting available as you say, see if you can change the CT with one of higher ratio..rather like fooling the relay, but what else to do?

Please calrify if it is CAG Relay or CDG Relay in the Incomer (English Electric Make); as of I know, you cannot set TMS in CAG Relay which is instantaneous.

Jamming in a pump can be prevented by using a filter or a strainer in the line upstream of it, so that the detritus that causes the jamming doesn't enter the pump volute.

Try using proper class of Thermal Overload Relay.