Overcurrent Relays

Earth faults can cause fatal harm to humans, even loss of life. Spend whatever you might, it is impossible to bring back a life lost.Isn't it? So, earth fault relays are mandated in all electrical installations.

Over current/Short circuit faults might cause harm to the property/installation, which might be resurrected by spending some money. The choice of having over current protection or not - hence - is decided by the person's willingness to take a risk to allow his installation/property to get damaged during a overcurrent fault.

There is no rule or limit to have or not to have overfcurrent protection below 300A feeders. I have come across overcurrent protection in feeders as low as 16A. So, there is no hard and fast rule on that.

In our factory the over current protections are there for the lower currents too (ACB tripping mechanism)

The ACBs are on Lines are as high as 800A FLC to some of less than 100A FLC.

I have experienced, that contractor designed the MCC low voltage feeder which has 100A breakers and less are not provided with specific earth fault relay. System is implementing solid grounded.

The reason (contractor explained) is that the groundfault current is detected by breaker, therefore no need RCD. Are there any opinion?

That depends upon the system of earthing. If it is TN (both TN-C & TN-S) - then you don't need any seperate RCD as any earth fault would be phase-neutral short and a very high current would flow such that any overcurrent protective device or short circuit protective device itself would identify and isolate the supply to the system.

If it is TT then you definitely need an RCD because the magnitude of the earth fault current now would be depending upon the earth resistance and would be in ohms. As such any over current protective device or short circuit protective device may not sense this fault and the system would continue to be faulty even causing a probable danger to human life and/or property. So, an RCD is mandatory in such systems.