If you are new to the industrial electrical industry, you’ll likely run up against a question that sounds like this the first time you order a disconnect switch:
Do you need fusing with that?
It sounds like a simple question but may stop you dead in your track. In this post, we will discuss the considerations and whether you require a fused or a non fused disconnect switch.
As always, we subscribe to the KISS (keep it simple, stupid) principle. So, while there are a lot of ways to go at this, we’ll start by just breaking down the basics.
- In an typical industrial electrical system, electrical power is 3 phase (and usually 480v)
- Fusing is typically provided in a circuit breaker (or set of circuit breakers)
- Most local disconnects are non fused disconnects and there’s probably an even split between a rotary disconnect and a knife blade style disconnect
- Non fused disconnect switches often have very limited short circuit withstand capability.
- A non fused disconnect generally has a short circuit withstand of 5000A, and a fused disconnect often has the withstand rating of the fuse that is inserted and approved by the manufacturer. This typically results in almost 200K and plenty of protection
The Determinants Of When Fusing is Required
Now that the basics are covered, let’s dive a little deeper. Here are three tips on when fusing may be required in a local rotary motor disconnect:
- For systems with high short circuit current, it is often necessary to use fused switches, just for the fuse to protect the switch
- A fused disconnect will also help with certain downstream withstand ratings, such as for some transfer switches
- On nearly every 480 volt system, the interrupting current is over 10kA, so fused should be used unless it’s protected by an upstream dedicated fuse sized to protect that switch/circuit
Now, you may notice that a few statements here could be contradictory. For example,
In nearly every 480 volt system, the interrupting current is over 10kA, so fused should be used unless it’s protected by an upstream dedicated fuse sized to protect that switch/circuit
A non fused disconnect generally has a short circuit withstand of 5000A, and a fused disconnect often has the withstand rating of the fuse that is inserted and approved by the manufacturer. This typically results in almost 200K and plenty of protection
The key words here are “dedicated fuse sized to protect that switch”. Usually, that protection exists upstream in an industrial electrical circuit. This would explain why a typical 3 phase, 480V circuit would have protection built in instead of being required at the switch itself. It would also explain why there are many more non fused disconnects sold than fused disconnects.
What Do They Look Like?
So, what happens if you didn’t order the product and need to determine whether the unit is fused? Well, there’s two simple ways to find out:
- Look at the labels in the product. It should be clearly marked
- If the label has worn off, it’s pretty easy to tell visually what kind of unit you are dealing with
Here is what a fused disconnect switch looks like:
Can you see the three cigar-looking things? Those are the fuses!
Here is what a non fused disconnect switch looks like:
Pretty much the same thing, except no area to install fuses.
Do I Pay More For Non Fused Disconnect than Fused?
Yes. For an apple to apples comparison, let’s look at a 30A Hubbell non metallic, non fused rotary disconnect switch from our friends at www.Zoro.com…
…and then compare is to a fused disconnect
The price difference is almost shocking, but that’s the market dynamic these days. Expect to pay at least 2x for the same switch if you require fusing/overcurrent protection.
Summing It Up
You should now know when a fused vs. non fused disconnect is required, what they look like, and how much more you’ll have to pay for fusing. What else did we miss? Let us know in the comments below!