Grounding Transformer Protection NEC 450.5

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JackyJohnson

Member
Location
Illinois
NEC allows the time delay for the grounding transformer tripping. Does it at maximum fault current with maximum withstand time rating ? How does TCC include operation from 125% over current to maximum fault current ?
 

beanland

Senior Member
Location
Vancouver, WA
Clarify

Clarify

Your questions are cryptic and poorly worded, can you clarify? For a grounding transformer, the 125% overload applies. So, continuous operation over 125% is not allowed. However, a grounding transformer is intended to supply a lot of current during a fault elsewhere on the distribution system, as such, the OCPD must be delayed to permit the grounding transformer to do its job. A MCB with magnetic protection may have a problem allowing the short term fault current. An electronic LSI breaker has the same issue unless the settings are correctly set so that downstream breakers have time to operate at the current sourced by the grounding transformer. A full system model and study is needed.
 

JackyJohnson

Member
Location
Illinois
Sometimes people set their relays at maximum fault current on TCC

Sometimes people set their relays at maximum fault current on TCC

Your questions are cryptic and poorly worded, can you clarify? For a grounding transformer, the 125% overload applies. So, continuous operation over 125% is not allowed. However, a grounding transformer is intended to supply a lot of current during a fault elsewhere on the distribution system, as such, the OCPD must be delayed to permit the grounding transformer to do its job. A MCB with magnetic protection may have a problem allowing the short term fault current. An electronic LSI breaker has the same issue unless the settings are correctly set so that downstream breakers have time to operate at the current sourced by the grounding transformer. A full system model and study is needed.

The reason I was surprised is because I came to few occasions where people set the relay at maximum fault current with maximum fault withstand capacity of the grounding transformers and that scared me. Thought of clarifying. It makes sense to have it set at 125% overload with time delay. Thanks.
 
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