Fuses marked "300VAC to Ground"

[derekk]

Hello, I'm wondering if someone can help me with this. I've always understood 240.60(A) to not allow a 300V fuse on a 480Y/277V system unless it's protecting a single-phase (L-N) circuit, per 240.60(A)(2). Today one of my foremen (who was performing documentation fin order for us to produce single line diagrams) sent me some pictures and I encountered something I hadn't seen before. Most of the blocks in the fusible 3-phase meter mains (200A to 600A) have 300VAC Class T fuses. I consider this a code violation. However, what I found interesting is that many of the fuses do not say, "300 VAC" and instead say, "300 VAC to Ground." This is interesting as a 480Y/277V system IS less than 300VAC to Ground. I noted that this particular manufacturer still produces fuses using the exact same part number but they now say, "300VAC" which sync's up with the current NEC.

Does anyone know if the NEC has changed with respect to this situation? I was going to ask the MFG on Monday, but I also thought perhaps someone here would know. I can't really advise replacement of dozens of fuses if they were code compliant when they were originally installed. Thanks in advance.

 

[big john]

I don't recall seeing fuses designated "to ground" but it still sounds like an incorrect application:

If the idea is that the fuse can only safely interrupt rated current at 300V or less, then it could never be used in a system where that voltage could be exceeded. In a phase-to-phase 480V fault, unless both fuses blew at the exact same time the first fuse would indeed be interrupting more energy than designed.

The way circuit breakers get around the "to ground" rating is that all poles will open simultaneously, so each pole contributes to clearing a portion of the arc.

 

[derekk]

The MFG is Littelfuse. It was just a very odd marking to have on a fuse. What's interesting is that labeling something with a voltage to ground implies that higher P-P voltages would be permitted. It makes me wonder if there was a code difference before my time and this fuse was designed and permitted to be used in a system such as 480Y/277V but not in a 480V Delta (for example). It doesn't really make sense, of course, but that labeling just strikes me as very odd.

 

[derekk]

I'll include this in case anyone else ever searches on something similar.

Apparently there was a code change in the past (I haven't researched when it happened). Littelfuse was able to read a note to me from their old literature that stated, "JLLN Class T 300 volt fuses are UL Listed for circuits not exceeding 300 volts to ground. However, since UL does not include testing 300 volt Class T fuses on 277/480 volt three-phase bolted faults, Littelfuse does not recommend using 300 volt Class T fuses where phase-to-phase voltage exceeds 300 volts." This is no longer part of their literature and these fuses are now labeled and listed as 300VAC.

The switchgear is Square D and the 200A fusible draw outs are still made (same part number) but their listing has changed as well. The current ones only go up to 240V. The labeling on the old switchgear stated that they were usable for 480Y/277VAC and 240VAC. It also stated NOT to use them on 480V, 3P, 3W delta systems, so those fuses were used in a manner that doesn't comply with current NEC (but apparently was permitted in the past). Since 600V fuses won't fit these draw outs, there is no provision to use 600V fuses in this installation. Good to know.