How to achieve power supply reliability

The increase in the use of field-mounted equipment that requires a reliable power supply reinforces the need for power supply equipment and designs. Unfortunately, when I talk to companies that normally provide AC/DC converters or UPS supplies, few of them provide the option of a unit suitable for installation any place other than the confines of a nice clean interface room, or at most an unclassified area.

The reality is that remote I/O, wireless access points and similar equipment are typically mounted in Class I Division 2 (Zone 2) classified areas.

So why are there so few options on the market that meet the requirements for reliable field mounting? First, I should explain what I feel a reliable field-mounted power supply should contain, besides, of course, the AC or 24 VDC power:
• Redundancy. The power supply should be redundant so that if one unit fails, there is no interruption in the output. As is done with most Foundation Fieldbus power supplies, this will likely mean a load-sharing configuration sized for 120 percent or better of load to handle surges during switch over and while repairs are being made.
• Separate mains. Each of the power supplies should be fed from two separate mains. The majority of larger facilities have an A and B bus in the plant so that maintenance can be done on the electrical system without requiring a full plant outage.
• Minimal points of failure. To provide the highest level of reliability, there should be minimal single points of failure, so the backplane for the units should not have any active components that could possibly fail. Any electronics should be mounted on pluggable modules that reside on this passive backplane so that they can be both easily serviced and upgraded in the future.
• Component failure. Some form of component failure needs to be included that can annunciate both locally and remotely via the control system. The remote indication can be either a single contact, a fieldbus signal or a wireless transmission to the DCS.
• UPS capability. The system should have UPS capability to not only clean up any noise in the resulting power output, but also to provide time for the components being powered from this supply to fail in a graceful manner, or at least give the operators time to take the appropriate action. Of course with the redundancy above, the batteries for the UPS will not be stressed too often, so the system should also include some way of testing them on a regular basis.

I am aware of only two options that meet the majority of these requirements — one from Weidmueller and the other from Phoenix Contact. The only reason I can think of for this limited number of options is that, until recently, there have not been sufficient potential applications to justify the development costs. It’s also possible that there are more products that exist, but that they are hidden in the product line.

Power over Ethernet (PoE)
An alternative to the field-mountable power supply is PoE, which can now generate a maximum of 56 watts of power at 40 VDC. Unfortunately, Ethernet is limited to distances of 100 metres, so in some cases, this will not be enough. Fortunately, most home run cables in a typical plant are just over 100 metres, so with careful planning this may be feasible. Alternately, if manufacturers were to make an Ethernet cable with larger wire size to reduce resistance, it may be possible to get longer distances.

Benefits
Having a field-mountable power supply or a PoE-enabled product suitable for field mounting on the market not only removes the hurdle to reliable field power, but when sized properly, it also gives you sufficient time to sort out the source of other operational problems that could be occurring at the same time, since you will now be able to monitor them. Better yet, it will give you the time required to bring your facility down in a safe manner.

This column originally appeared in the May 2012 issue of Manufacturing AUTOMATION.