Isolation Transformer Questions

I have a 7KVA isolation transformer and having issues where it's tripping the circuit breakers. I have two circuit breakers, one main and one acting as a cutoff switch both installed on the primary side. Transformer manufacturer specification says 235A of inrush current with 7KVA purely resistive load. I have two questions:

1. I am fearful to load the transformer with equipment, so will the load on the secondary side see the inrush at all?

2. If yes to question 1, is there a way to calculate what the anticipated inrush is assuming we know our load (for purely resistive)? I presume it would be a little more complex for inductive or capacitive loads. I do know based on research that it is lower when load is applied, just don't know how much.

Answer: Your Load on the secondary side will not see any inrush current. You don't have to worry about loads on secondary side.

I suspect your isolation transformer is a single phase, rated at 240V, which has a primary current rating of 30A. For such small transformers an inrush of 235A (8 times nominal current) is reasonable. I suspect you are using miniature circuit breaker (MCB) to protect the transformer.

If that's the case, then I suspect, the miniature circuit breaker is a 'C' type breaker. Such breakers have a fixed magnetic trip setting of 5 to 10 times thermal rating. This means if the current through the MCB is between 5 to 10 times the rated current, the breaker will trip instantly. All that you have to do is simply change this to a 'D' type breaker with similar thermal rating. 'D' type MCB's have an instant magnetic trip setting between 10-20 times the rated current.

NOTE: You must not use RCBO (residual Current Circuit Breakers) or ELCB (Earth leakage circuit breakers) when providing power to transformers. The inrush will inevitable trip such devices, unless they have a time delay element.

If you are using a circuit breaker with shunt trip coil and external over current relay (extremely rare for such small transformers), you have to set the instantaneous setting to over 8 times. Alternatively relays are available that can implement 2nd harmonic blocking, which detects transformer inrush.

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