Ups Control Cabinet

Where to mount the uninterruptible power supply in the cabinet

May 1, 2023
The control panel is prime real estate, but critical components need their space

What’s in a control panel? The size of the panel may give us some indication as to the complexity of the application.

A 20-by-24-inch panel probably won’t have much in it, but what it should have is often forgotten.

A 120-by-80 would have considerably more devices in it, which would require more of the things that are often forgotten. What might those things be?

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Back in the good old days, we had power filters, capacitive filters, metal oxide varistors (MOVs), coil suppression and free-wheeling diodes on dc coils. We had no indication that anything actually worked until you opened the panel because a coil has gone, and you notice that the red MOV is now brown. Passive devices are still very relevant and shouldn’t be forgotten.

An inductive load when power is removed kicks back. Larger inductive devices need to be especially not forgotten.

In this day and age, the main control power normally would go through an uninterruptible power supply (UPS). There is an active discussion about what a UPS should connect, but that’s another story.

Uninterruptible-power-supply sizes range from 200 VA up to 1,500 kW. Application of each boundary is totally different. In most cases we are talking about 200 VA to 3,000 VA located in the control panel itself.

The input to the UPS is typically 120 Vac, and the output is 120 Vac, which means that the control power is filtered and supplied by the UPS, and, when main power is removed, the UPS provides an approximate sine wave, or true sine wave, to the controls in the cabinet.

In most cases, when the UPS kicks in, power has been lost, which typically also means the main power—480/600—is also gone. The situation that exists now is that the contactor that feeds the main conveyor belt is still energized, but the motor isn’t turning, which needs to be taken into account in the shutdown sequence(s) of the application.

Uninterruptible power supplies can create their own issues. Should the UPS fail, the output sometimes can be turned off permanently, and, no matter what you do, you can’t get it back. Also, should the UPS turn off due to battery exhaustion, it may enter a mode where the UPS may need to be cycled for the UPS to turn the output side back on, requiring manual intervention.

Many sites use commercial-grade UPS devices. Are they inexpensive? Yes, and for the most part reliable, but it would still be suggested to have some up-front filtering.

There was a customer who had the UPS sitting in the bottom of the panel in a toxic-waste-clarifying facility. The panel had conduits coming into the top of the panel from outside. The conduit had a loose joint where water was allowed to enter. Yep, the bottom of the panel was swimming in 3 inches of water, just below the first outlet on the back of the UPS.

So, it begs the question: Where should a UPS be mounted in a control panel if there is a choice? I think it is clear that the bottom of panel would not be the best place in most instances.

Heat dissipation and ambient temperature are important considerations, which suggest that mounting at the top of the panel may not be the best option either. So where does that leave us?

Since manual intervention may be required, it needs to be accessible to maintenance, which is normally in the front panel. The health of the UPS may need to be visible from the front panel.

The form factor of the device may make the mounting decision a moot point. A rack-mounted UPS would dictate location—the rack. A shelf may also be needed to support a non-rack UPS.

One of the biggest potential benefits of any UPS is the ability to be monitored remotely using Ethernet and/or digital interfaces. This would allow the UPS to indicate what it’s doing internally and what function it is performing in real time.

In effect, we need to position the UPS to protect itself—accessible to the front panel and to the rear for plugs and safe from environmental issues. These are the main considerations. In checking a 1,000 VA UPS that is 2U—3.5 inches high—and 18.8 inches in depth, this particular unit has six plugs in the back plus a power cord that requires some clearance.

A standard panel depth is typically not more than 20 inches. A normal 1,500-VA shelf-mount UPS is 15 inches deep so it would typically need to be mounted sideways, which compromises access to the front display panel.

Once a UPS size is determined, the mounting of the device may become apparent due to mechanical constraints, but it should still be thought of along with those things that should not be forgotten.        

About the Author

Jeremy Pollard | CET

Jeremy Pollard, CET, has been writing about technology and software issues for many years. Pollard has been involved in control system programming and training for more than 25 years.