Fuse rating on lockout relay circuit
Fuse rating on lockout relay circuit
(OP)
I'm trying to size the fuse protecting a lockout relay connected to a generator protection relay (generator rated at 13.8kV). The current design uses 14AWG wires with 30Amp fuses. Circuit is wired to a 125VDC source. When operated the lockout relay draws 6-7amps.
However as I was reviewing NEC codes section 240.4, 310.15, 310.16, I believe the right size to use is 15Amp. This sort of tells me that 30Amp is oversized.
Here are the considerations that I've received from different sources that say 30Amp fuse is okay:
1. The 14AWG wire only sees current when the lockout relay picks up during a fault. Because it's not used as a continuous current carrying wire, it does not violate NEC standard.
2. Because the lockout relay is used to protect the genearator, the oversized 30amp fuse is used to accept most short circuit faults. Even if the short circuit burns out the rely coil, it's an acceptable risk since it is used to protect the generator.
Any comments would be helpful.
Thanks.
However as I was reviewing NEC codes section 240.4, 310.15, 310.16, I believe the right size to use is 15Amp. This sort of tells me that 30Amp is oversized.
Here are the considerations that I've received from different sources that say 30Amp fuse is okay:
1. The 14AWG wire only sees current when the lockout relay picks up during a fault. Because it's not used as a continuous current carrying wire, it does not violate NEC standard.
2. Because the lockout relay is used to protect the genearator, the oversized 30amp fuse is used to accept most short circuit faults. Even if the short circuit burns out the rely coil, it's an acceptable risk since it is used to protect the generator.
Any comments would be helpful.
Thanks.






RE: Fuse rating on lockout relay circuit
RE: Fuse rating on lockout relay circuit
As mentioned, there may be inrush current into the LOR coil.
30A is a common size fuse for 125V dc breaker control circuits. The fuses are there as much for isolation/maintenance as for any kind of circuit protection.
RE: Fuse rating on lockout relay circuit
My uneducated thought is that it sounds like 30A is unnecesary, does not protect the cabling, and 15A would be better. In either case there should be means to tell if the fuse has blown, usually a red light indicating continuity of that path through the 86 coil. That gives some assurance that your protection won't be compromised by loss of power to the 86 provided there are operators around.
RE: Fuse rating on lockout relay circuit
Sometimes strict code compliance is demanded, where the AHJ is more familiar with “premises wiring” systems.
He may more readily buy into 10AWG conductors {type SIS within gear… RHW/XHHW externally} accompanying the larger fuse size. One boilerplate practice in circuit breaker DC-trip circuits is using 35-amp fuses and 10-gauge conductors. Then, there’s the matter of every inch of the subject wiring being grey…
A white indicator lamp [with integral series resistor] can be placed in parallel with all the contacts that initiate 86 operation. This is a simple form of “supervision”—indicating that station-battery voltage and lockout-relay trip-coil continuity are OK.
RE: Fuse rating on lockout relay circuit
RE: Fuse rating on lockout relay circuit
That's the way ours is, anyway. No doubt there are many approaches.
RE: Fuse rating on lockout relay circuit
By the way, our fuses are 20A. Not sure the wire size or whether these are intended to provide overcurrent or short circuit protection.
RE: Fuse rating on lockout relay circuit
RE: Fuse rating on lockout relay circuit
I'll bet you also have DC motors on oil pumps with no overload/overcurrent protection ...
RE: Fuse rating on lockout relay circuit
A useful reference may be Relay Trip Circuit Design by the IEEE-PES Power Systems Relaying Committee.