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To Fuse or Not To Fuse
2

To Fuse or Not To Fuse

To Fuse or Not To Fuse

(OP)
Hello Everyone,

I am currently charged with decommissioning a 15kV ckt bkr that is in series with another 15kV breaker.  The ckt bkr to be decommissioned is mainly a disconnecting means.  The XFMR has a neutral grounding resistor with a set of CT's that are connected to a 51G and an 86 relay that interfaces with the breaker that the owner wants to decommission.  The owner would like to pull a new feeder from the upstream 15kV breaker that has the relaying to a disconnect switch on the primary of a 7500kVA 13.8kV-2400V Cast Coil XFMR.  My question is does the new disconnect need to be fused?  The upstream breaker is interfaced with a 50/51 Relay that has a 600/5 CT connected to it.  I believe I am bound to NEC Table 450.3(a) for the Maximum Rating of Overcurrent Protection for Transformers over 600V.  My XFMR has a % Imp of 9.9 so therefore I have a supervised location with a XFMR % imp of more than 6 not more than 10% the chart leads me to 400% of the XFMR FLA.  I calculate the primary FLA to be 7500kVA/13.8kV*SQRT(3)=313.77A therefore the maximum size breaker is 400% * 313.77 = 1255.10A.  The upstream breaker is a 1200A breaker that is relayed with a 600/5 CT into a 50/51.  Does this mean I don't need fuses?  It appears I need to pull the 51G/86 relay signal to the new breaker as well.  Any advice with this situation is greatly appreciated.  Thanks in advance.  

RE: To Fuse or Not To Fuse

Use breaker & relaying only, IF you have compliant secondary overcurrent protection with T450.3(A) note 2.

Note this transformer, at 5MVA, is recommended by IEEE Buff Book to have 87T/86T differential protection and 63FP/63X rapid pressure rise protection, also.

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