Fuse rating on lockout relay circuit

[freiwill01]

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.

 

[jbartos]

Suggestion: The lockout relay manufacturer might be contacted for the lockout relay recommended protection. There may be an inrush current, when the lockout relay operates. The 30A fuse is unsafe if any undesigned voltage is impressed on the circuit or some stray path for the current, larger than the #14 conductor is rated for, develops. Certain types of shorts or malfunctions are not fully predictable.

 

[dpc]

I'd stick with the 30A fuses. What is a 15A fuse going to protect against that a 30A fuse will not, and is more important than tripping your generator?

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.

 

[electricpete]

The 86 coild should have N.O. contacts wired in series with coil so it won't be continuously eneergized and burn itself out (regardless of fuse protection).

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.

 

[busbar]

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.

 

[gigahertz]

If you're looking at NEC issues, 240-3(a) specifically requires NOT installing overcurrent protection where an open circuit will create a hazard and I have always seen this rule applied for protection circuits of large generators. The circuit requires short circuit protection only, which is frequently accomplished by oversizing fuses.

 

[electricpete]

We have 2 1250MW generators, which I think qualifies as large. The 86 relay circuits are fused. But there is redundant protection in separate circuits (separate fuses). If one 86 is disabled by blown fuse, a backup will accomplish the trip (and opertors will be alerted by absense of a light). On the other hand as mentioned if short circuit fuse protection is not provided, a problem in one circuit can affect multiple circuits.

That's the way ours is, anyway. No doubt there are many approaches.

 

[electricpete]

When I re-read the previous posts I see I wasn't saying anything new/different than Ghz on the subject of how large generator protection. It's not just the existence of fuses but the purpose that's important.

By the way, our fuses are 20A. Not sure the wire size or whether these are intended to provide overcurrent or short circuit protection.

 

[jbartos]

Suggestion: Substations have #12AWG common. Also, 20A protection for #12AWG is common. Therefore, the LOR (ANSI Dev. No. 86) will be protected by 20A fuse and its coil trip circuit supervised by Relay ANSI Dev. No. 30.

 

[gigahertz]

Electricpete:
I'll bet you also have DC motors on oil pumps with no overload/overcurrent protection ...

 

[busbar]

A useful reference may be Relay Trip Circuit Design by the IEEE-PES Power Systems Relaying Committee.