Current Injection

Er,,when you say 'secondary injection', you must mean that you are passing a current through the relay which is normally done by the seconday of the CT...yes? If so, that current is likely to be a percentage of 5A usually, if the CT has a X/5 ratio. When you say 350A, do you mean that you have multiplied the actual current passed by the ratio?

OK, assuming you have done that, has your source sufficient power to drive the current through the relay? Is there a heater in the circuit which increases in resistance as it heats up (quite normal in bimetallic elements) thereby reducing the current flowing through it?

One or more than one cable joints have unacceptable levels of resistance.

Possible, even likely.

But then, why does the current drop after some time? Could be aggravation of the loose contact, or risng resistance somewhere, maybe even internal to a weak source.

The heating of the windings will increase their resistance, causing a decrease in current drawn. I don't know if this effect is enough to account for the reported decrease.

I meant to say wiring (etc.) rather than windings, but it became too late to edit.

What is the rated current of the CB,what do you mean by 630A ACA?. Why did you set OL at 252A and why you injected 350A in the primary?. What is the CT ratio?.

Well ,i figured out that problem was in the cables i.e the 35mm2 cables from injector to a.c.b which overheated extreemly at such high current .so i used larger leads of 95mm2 and problem was solved.just sharing my experience.

Ahh..so you mean PRIMARY injection, don't you?