Code violations: UL98 versus UL508 switches

Source: PlantServices.com

By Roger Pollack

Apr 02, 2008

A simple and reliable means for disconnecting power to motor loads is the Underwriters Laboratories-listed UL98 safety switch, but UL508 manual motor controllers suitable for motor disconnect have gained interest recently because of changes to the National Electrical Code (NEC).

However, with that interest has come concerns about possible misapplication of UL508 motor controller switches, which can result in added cost and time to replace with the correct switch, leading to downtime and missed deadlines. Informal surveys suggest that misapplication of UL508 products in UL98 applications is substantial. One such survey of electrical contractors, consulting engineers and OEMs at a recent Schneider Electric customer event indicated that about 40% had misapplied UL508 motor controller switches as general-use switches.

Thus, understanding the key differences between UL98 and UL508 devices and correct applications for each can help an industrial facility manager ensure an installation is NEC-compliant and avoid downtime, lost sales and customer frustration.

Understand the basics


Both UL508 and UL98 switches are available in traditional and receptacle interlock versions. A traditional switch is hard-wired to a fixed electrical load – an electrical contractor must run conduit to the load from the switch, making it a permanent attachment. A traditional, hard-wired UL98 safety switch is easy to spot – it features  a large handle on its right side that disconnects the power when pulled.

A switch with a receptacle interlock allows machines to be relocated when necessary, meaning they don’t take up space that can be used for other purposes. A UL508 switch with receptacle interlock typically features a rotating on-off dial, along with a cord and receptacle into which a machine is plugged. A key safety feature of receptacle interlock switches (including UL98 devices) is a design that forces an operator to turn off the load before removing the machine’s plug from the receptacle.

The ability of a UL508 manual motor controller, particularly the receptacle interlock version, to disconnect a motor while not affecting the main circuit can be a great asset that keeps processes running. However, using a UL508 switch as a branch-circuit disconnect or for non-motor loads is a violation of the NEC.

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Avoid misapplications

There have been concerns in the industry regarding misapplications in those ways. 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 guideline, 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 motor-powered machines, which is more desirable than a UL508 misapplication that could cost time and money to rectify.

Roger Pollack serves as the product line manager of safety switches for Schneider Electric.

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