Transformer primary only protection

[art]

How does it protect the secondary?

 

[petersonra]

the amount of current at the secondary is the ratio of the number of turns to the current in the primary. so if there is an overcurrent in the secondary it will be reflected to the primary and the primary OCPD can open.

it doesn't work if there is a neutral involved but as long as it is a delta system or a 2 wire single phase system it works,

personally I don't like it because you have to protect the primary at a relatively low level (125%) and sometimes that can cause nuisance tripping.

 

[Dale001289]

How does it protect the secondary?

Say you have a 480-240V Xfmr with two wire primary winding (and circuit breaker) at 50A with a two wire secondary wound for 100A - you'd have 1:2 winding ratio. So when the secondary sees 100A, the primary sees it as 50A and opens - which is acceptable per NEC. However if a neutral is provided on the secondary for a three wire condition you'd have 480-240/120V and a 1:4 ratio.
If you have all secondary circuits loaded at 120V, means up to 200A will be on the secondary before the primary over current device sees it as 50A and opens. NEC finds this unacceptable due to the heat generated on secondary conductors before the primary side circuit breaker opens. Hence the need for secondary protection.

 

[art]

Thank you both for the detailed and clear explanation

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