What Disconnect Should I Use?

The first thing is to see if your control panel will need to have a disconnect switch based on the applicable standards.

The requirements for a disconnecting means (operating handle) in an electrical control panel classified as Industrial Machinery are found in NFPA79, the Electrical Standard for Industrial Machinery, and UL508A, the standard for Industrial Control Panels, Section 65 and 66, dealing with Industrial Machinery applications. 


What is an Industrial Machine?
According to NFPA79 and UL508A, the following types of machines are classified as Industrial
Machinery:

  • Metalworking machine tools
  • Plastics machinery
  • Wood working machinery
  • Assembly machines
  • Material handling machines
  • Inspection and testing machines


What is not an Industrial Machine?
The following types of machines are not classified as Industrial Machinery.

  • HVAC
  • Pumps
  • Fans
  • Wastewater treatment
  • Portable machines of any kind.

 

What is a Disconnecting Means?

 According to NFPA79 and UL508A, we can summarize the following:

  • Disconnect = UL98 Listed General Purpose Switch, UL489 Molded Case Circuit Breaker, or UL489 Molded Case Switch.
  • Disconnecting means = operating handle of the disconnect.
  • These operating handle requirements only apply to the main disconnecting means supplying power to the industrial machine.
  • A UL508 Manual Motor Controller cannot serve as the main disconnect to an industrial machine and is not subject to these requirements.

Below are the UL508A and NFPA79 overlapping standards for disconnect requirements in control panels:

NFPA79 - 5.3.3.1 (3) Disconnect mechanism shall be padlockable in the “OFF” position only, independent of the door position.

UL508A - 66.6.3 (d) Able to be locked in the “OFF” position independent of the door position; and when locked, closing of the disconnect is not possible.
 
NFPA79 - 5.3.3.1 (5) Disconnect mechanism operable independent of the door position 
without the use of tools.
 
UL508A - 66.6.3. (c) Be operable independent of the door position without the use of accessory tools or devices;
 
NFPA79 - 5.3.4.2 (1) Operating handle accessible with door in the open or closed position.
 
UL508A - 66.6.3. (a) Readily accessible when the enclosure doors are in the open or closed position;
 
UL508A - 66.6.3 (b) Installed so that its operation is not restricted by the enclosure door while in the open position;
 
NFPA79 - 6.2.3.1 Door interlocked with operating mechanism in “ON” position.
 
UL508A - 66.1.5 A door of an enclosure that gives access to uninsulated live parts operating at 50 volts rms ac or 60V dc or more shall be interlocked with the disconnecting means such that none of the doors can be opened unless the power is disconnected.
 
NFPA79 - 6.2.3.1.1 Door interlock defeatable in “ON” position (optional).
 
NFPA79 - 6.2.3.1.2 (1) Tool required to defeat door interlock when switch on (door closed)
 
NFPA79 - 6.2.3.1.2 (2) Door interlock reactivates automatically when door is closed.
 
UL508A - 66.1.5.1 (a) Means to defeat the interlock without removing power and which 
requires the use of a tool to operate;
 
UL508A - 66.1.5.1 (c) Reactivated automatically when all the doors are closed
 
NFPA79 - 6.2.3.1.2 (3) With door open, switch may only be turned on by defeating a
mechanism interlock with “deliberate action” (no tool required).

UL508A - 66.1.5.1 (b) means to prevent restoring power while the enclosure doors are
open unless a defeat mechanism is operated; 
 
 
What do all these standards mean?
It means that based on the classifications above if your control panel is an 'industrial machine' it does need a disconnect.  As part of your design then you should specify to the panel maker that the disconnect must comply with UL508A and NFPA79. 
  

One of the things that you have to now start to consider is what are the power requirements for my panel? Are you going to have motor starters or VFD's inside the Panel that require 3-phase power? 

You should start making a Bill Of Materials up for your panel and the power requirements for all the components that are going inside.  This will allow you to add up all the power required and size the disconnect accordingly.

Control Panel Disconnects can be broken into at least four types:

  • FLANGE 
  • THROUGH DOOR (Rotary)
  • ARC FLASH MITIGATION
  • EXTERNAL