Understanding Two Main Types of Motor Disconnects

In the industrial electrical industry, the word “disconnect switch” gets thrown around quite frequently. While it’s true that a disconnect switch serves the same general electrical function no matter what, it’s important to understand the context of a disconnect switch and its applications as well as the different UL ratings associated. In this article, we will cover the two main areas of disconnect switches:  branch disconnect switches and local motor disconnects. However, before we begin, it’s important to note that we are defining a disconnect switch as both the switch and the enclosure that houses the switch. We are doing so because in our experience, most industrial customers are buying a switch and enclosure in one.

Branch Circuit Disconnects (UL 98)

Simply put, branch disconnects are meant to be a means of disconnecting an electrical load at the main power source. The most typical version that is seen in the field are the large “knifeblade” style disconnects made by companies such as ABB, Eaton, or Square D that look like this:

Square D UL98 disconnect

Because they are located far upstream in the circuit, it’s not uncommon for these disconnects to support extremely high amperage ratings.For example, Eaton sells UL 98 disconnect switches up to 800A.  Also, the SCCR (short circuit current rating) is higher than the local motor disconnects and  can support up to 200 kA.

Additionally, a UL 98 branch disconnect switch can be a part of a combination starter unit. Below is an example of the various components of a combination starter unit and where the branch disconnect fits in the electrical order of operations:

Combination starter diagram for motor disconnects

Whether the UL 98 branch circuit disconnects are part of a combination starter or are contained in a standalone enclosure, it is quite frequent that the branch motor disconnects are fused. This is the case because it is advantageous to protect all of the motors and equipment that the circuit is powering as far upstream as possible. Thus, fusing at the branch circuit level is more efficient than having various fuses further downstream (although there are, of course, always exceptions and reasons why this cannot always be the case). If you look at the figure above, you can see that the “Motor Branch Circuit Protection” is essentially the fusing and it can be incorporated either within the disconnect or outside of it.

Local Motor Disconnects (UL 508)

Now that we have covered branch disconnects, let’s break down the characteristics of a local motor disconnect under the UL 508 standard for devices and components intended to be used for starting, stopping, regulating, controlling, or protecting electric motors and industrial equipment.

In 2002, the NEC made a change that impacted the standard  and application of these types of disconnects. While we try to refrain from being technical, it’s important to mention that it was Section 430.102 that provided the requirements for understanding when a disconnect must be used. Specifically, an individual disconnect must be installed for each motor controller and that disconnect must be in sight from the motor controller. To translate that as simply as possible, every motor has to have a local disconnect and it has to be close to where the motor is controlled.  The biggest key phrase in this requirement is the phrase “in sight from” and it is defined in the following way by the NEC in Article 100:

Where this Code specifies that one equipment shall be ‘in sight from’ another equipment, the specified equipment is to be visible and not more than 15 m (50 ft) distant from the other.

Now that we have the proper context behind why local motor disconnects are needed, it’s possible to understand the key features of the product characteristics that exist in the market. While branch circuit disconnect can quite frequently support extreme amperages, it’s more common in industrial environments that local motor disconnects support lower amperages. The most popular amperage ranges would be 20-200A. Because the amperage is lower, the enclosure that are needed to contain the switches are also smaller. And while UL 98 disconnect enclosures are typically metallic, the UL 98 local motor disconnect can have either a non-metallic or metallic enclosure. The enclosure type is typically dictated by the environment in which the units are installed. Here are two examples of what a local motor disconnects would look like:

mennekes ul 508 motor disconnects

 

Also, the majority of local motor disconnects are non-fused for the simple reason that fusing is typically already provided at the branch circuit and additional fusing would be overkill. However, in circumstances where that is not the case for whatever reason or there are special circumstances, local motor disconnects have a fusible option as well. Beyond fusing, local disconnects typically have auxiliary contacts to wire back to a VFD (Variable Frequency Drive) or PLC (Programmable Logic Controller) and are HorsePower Rated based upon the possibility that the switch will be used to break the direct motor power under load.

They Sound Similar: Are They Interchangeable?

No. While electrically the branch circuit disconnect switches and local motor disconnects serve a similar function, using a UL 508 switch as a branch-circuit disconnect or for non-motor loads is a violation of the NEC. Schneider Electric conducted a survey indicating that about 40% had misapplied UL 508 motor controller switches as general-use switches. Here’s further insight from that same article:

For example, a branch circuit that powers machinery in a manufacturing facility can’t use a UL508 switch of any kind – either traditional hard-wired or receptacle interlock – to disconnect the entire circuit. A UL98 safety switch is appropriate in that situation, while a UL508 switch could be used to disconnect power to a motor within a branch circuit or on the machine (NEC 2008 430.109(6)).

There are many reasons for misapplication, but the most prominent is ignorance of the NEC restrictions on UL508 devices. Another is cost – a UL508 manual motor controller can be smaller and less expensive than a UL98 switch, and it might be tempting for an electrical contractor to install a UL508 device to keep costs down. The best guidance, then, is unless you feel confident that an electrical contractor understands the proper application of UL508 devices, insist on a UL98 safety switch. a UL98 switch will still cut the power to the main and branch circuits, including the motor-powered machines, which is more desirable than a UL508 misapplication that could cost time and money to rectify.

Summing it Up

By now, the differences between a UL98 branch circuit motor disconnect switch and a UL508 local motor disconnect should be quite clear. Here’s a quick hit list in case you skipped to the end:

  • Branch circuit motor disconnects (UL98) are upstream in the electrical circuit and local motor disconnects (UL 508) are further downstream
  • UL 98 disconnects are typically fused. UL 508 disconnects can be fused or non-fused
  • UL 98 disconnects support higher amperages, while UL 508 disconnects support lower amperages
  • There may be only a few UL 98 disconnects in a facility. There can be hundreds of UL 508 disconnects in that same facility

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