Understanding Local Motor Disconnects (UL 508)
Local Motor Disconnects (UL 508)
Let’s break down the characteristics of a local motor disconnect under the UL 508 standard for devices and components, which is intended to be used for starting, stopping, regulating, controlling, or protecting electric motors and industrial equipment.
Technically speaking, 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.
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 is not necessary. 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.
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