Service Entrance Switch necessary for Low Impedance 225 KVA padmount Transformer?

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Snowjob

Snowjob

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I am doing a review of an old factory site. I have a 225 KVA pad mount transformer with a Low impedance of X=1.5%. It's a Westinghouse unit sitting on a Pad.

Presently we are using 800 A Solid State Square D MX Micrologic Breakers to protect the transformer secondary. They are clearly labelled "Service Disconnect".

Everyone seems satisfied with this arrangement. However I mentioned this to a much more older experienced engineer and he stated that you should never do this with a low impedance transformer and that a Fused Service Entrance Switch is neccesary
.

What is the Technical Basis for this older engineer's statement?
 
Snowjob said:
What is the Technical Basis for this older engineer's statement?
Click to expand...
There is none.

Most likely, he is under the mistaken belief that only fuses can be used when fault currents are high. Because, with all other things equal, a low %Z yields a higher amount of short circuit amps.

In your case, a 225kVA transformer at 480V and 1.5%Z would have a fault current of about 18KA, and at 208V it would be 42kA. Both of these are well within the AIC rating of your breaker.
 
The Service Entrance switch..

The Service Entrance switch..

I am a former Electronics Engineer learning Electrical.
Which statement is correct? I've heard it different ways from different professionals.

1. The Service Entrance switch, whether it be a Fused Switch or a Solid State Breakers, is there to protect the factory (consumer) feeders downstream of the transformer secondary should there be a fault or overcurrent in the Transfomer secondary. Transformer secondary protection is the responsibility of the Utility.

2. The Service Entrance switch, whether it be a Fused Switch or a Solid State Breakers, is there to protect the Transformer secondary from a Fault inside the factory (by the consumer ).
 
Snowjob said:
I am a former Electronics Engineer learning Electrical.
Which statement is correct? I've heard it different ways from different professionals.

1. The Service Entrance switch, whether it be a Fused Switch or a Solid State Breakers, is there to protect the factory (consumer) feeders downstream of the transformer secondary should there be a fault or overcurrent in the Transfomer secondary. Transformer secondary protection is the responsibility of the Utility.

2. The Service Entrance switch, whether it be a Fused Switch or a Solid State Breakers, is there to protect the Transformer secondary from a Fault inside the factory (by the consumer ).
Click to expand...



this in not the only function, but it will provide this protection.
 
This is a separately derived system. The circuit breaker is both a disconnect and an overcurrent device. You are allowed to have a disconnect on the secondary side. If the transformer is the property of the customer than I don't see how the utility could be responsible for it's protection.
 
The Transformer belongs to the Utility

The Transformer belongs to the Utility

The Transformer belongs to the Utility. Does this change anything?
 
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