What is "Hot Wire Fusing"?

overloaded /improper connections/junctions which could be a source for ignition, especially in context of Div 1/Div2 atmospheres

Fast disconnecting fuse.

Common in Electrical industry.

Cheers,

Ethobil

Hmmmm.... thanks for the feedback folks. Interesting that the two responses are so different. Makes me wonder how common it really is. Not only that, but over 100 people have read this post and only two had answers. Wonder where UL found the reference......

As I do not have a copy of UL 913 available, these comments are germane to the discussion.

In the early days of my electrical career, as an apprentice, I worked in an old rotogravure printing plant. In this plant, the entire pressroom area was a class 1 div 1 location. The atmosphere was a Group IIA classified one and was pretty nasty.

We had 4 "press control rooms" that contained all of the motor and press control systems. The room was pressurized positive with respect to the adjoining hazardous area and had airlock doors to prevent the Group IIA atmosphere from entering.

Each room had a 6' high "wall" of open starters, fuses, knife switches, relays, timers and every other type of discreet component. Some of the "fuses" consisted of phenolic blocks with connection points for incoming and outgoing conductors. These connection points led to other screws via brass "jumpers". Between these second screws was a "fuse wire". The wire would melt and open the circuit as per a regular fuse. We used to have boxes of "calibrated wire" whose diameter were very closely controlled. These units were stripped out and replaced with cartridge fuses in the middle 70s.

Perhaps that is what the term "hot wire fusing" refers to in the UL standard you mention. Perhaps you could post the text of the clause that mentions these fuses.

I tried to find a photo of one of the old fuse block/holders on Google image but could not. Here is another image that conveys the idea. Replace the fuse body in this image with a piece of calibrated wire.

North of 60 !!!

You are right. Early fuses were "Hot Wire Fusing" & I remember a piece of rated wire was always contained in fuse-holder.

Later "Slyd-Lock" brand Fuses & Fuse Holder were all working like this.

Regards

UL913 is the Intrinsic Safety standard, and here's the reference:

8.1.1.5 In evaluating circuits for intrinsic safety, ignition sources such as the following are to be considered:
a) Sources of spark ignition:
_____1) Discharge of a capacitive circuit;
_____2) Interruption of an inductive circuit;
_____3) Intermittent making and breaking of a resistive circuit;
_____4) Hot wire fusing.
b) Sources of thermal ignition:
_____1) Heating of small gage wire strand;
_____2) Glowing of a filament;
_____3) High surface temperature of components.

I guess the question boils down to the difference between "hot wire fusing" and just "fusing".

Graebeard !!!

You are right as the safety measures adopted in Operation Theaters of Hospitals also follow these strictly as certain gases used are ignited if any open-electric-switch is operated. etc etc. So all such fixtures are specially made for these safety areas.

I think other fields may be Petroleum / Gas, Explosives etc etc installations.

Given the clause you posted, I would consider the term "hot wire fusing" to describe the method of conductor protection that I described. Where a "melting element" is exposed to the environment as opposed to a plug or cartridge fuse where the melting element is contained inside a body.

That does make sense, although makes you wonder about the ages of some who sat on the UL Technical Committee that wrote this. It's also tough to believe someone would design an I.S. piece of equipment with open fusing like that.

Anyway, thanks for your info. I'm going to go with your interpretation and think we can put this question and thread to rest.

If any late readers of this thread have something more to add, I'll still be interested.

Thanks everyone .....

North of 60 is absolutely correct.

Thank you

Ethobil

Hi folks,

Just to put closure on this, I finally received the goods from UL itself. The intent is that as a conductor gets hot (from overload), it can can melt or fuse, and in the process create arcs and sparks. This can lead to ignition of the surrounding combustible gas. So, it has nothing to do with fuses or wires designed to be fuses, but has to do with wires that behave like fuses and make like a firecracker.

Who wouldda thunk that's what they meant?

Anyway, just wanted to pass it on to you for your information and to say thanks for the input.

Now it's off to the pub ! (Yeah, I wish)