Overload vs Short Circuit

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As far as I can recall, no standard on protective relays has used the term short circuit with a protection perspective. All they say is only overcurrent. Moreover, these are relative terminologies. For a 1000A feeder, a fault current of 2000A may be just overload but for a 100A feeder, it could be short circuit. There are no clear demarcations. During an actual short circuit between two points at different potential, the short circuit current could be much less than any overload current, if the fault impeadnce is more.

1. Over load setting depend upon the equipment's over load capacity

You may refer IEC standards / manufacturer's rating specs.

Say, Transformers have over load capacity as

10% for 1 Hr. - Over load setting protection

50% for 30 min. - High set current setting of O/L relay

100% for 10 sec. - Instantenous current trip -

(As generally, transformer are with Short circuit capacity of 15-20 times Irated )

The overload relay should provide protection upto the breaking current capacity of the contactor and above that SCPD should clear the fault current.

This topic is labelled co-ordination. The protocol for doing it can be found in many electrical standards; BS7671 is applicable in the UK.

Overload pertains to the protection of the motor.

Short circuit protection is for the wiring feeding it.

Motor protection it is not a simply explanation. There are standards recommending how to proceed in each country. For instance, in US NEC art.430 [270 of sub articles in about 30 pages] indicates how to fit the protections [see Fig.430.1 attached here].In U.K. BS 7671 ,IEC 60034 and other.