When performing preventive maintenance on switchgear, particularly Infrared (IR) thermography, one needs to power down, open up, disassemble, and reenergize the switchgear. Then make measurements while it’s open! Scary! I haven’t spoken to a single engineer or electrician that looks forward to that moment. Or the time required in front of open cabinets to take the IR thermography shots. This mechanical intrusion into the switchgear, especially metal enclosed switchgear, removes all safety barriers designed into the switchgear just so the bus bars and the connections can be visible for an IR thermography shot. This mechanical intrusive, labor-intensive work while the switchgear is de-energized, can be the catalyst that changes a minor problem to an arc flash hazard. With all safety barriers removed, and the mechanical intrusion, the time of reenergizing the switchgear becomes a time of great concern, second only to the time the switchgear is re-energized after being completely reassembled and being put back online.

The arc flash threat itself is not predicated on the load current, but the condition of the mechanical connections of the bus bars and other connections, and the source impedance of the power feed. But the load current is critical to finding a marginally bad connection that will be a source of a failure later on that can grow into a potential arc flash hazard during another PM cycle.

Since we brought up the subject, just a word on loading... A load representing the normal operating load, or loading capacity needs to be used if one is spending all this labor to expose the switchgear (as well as time in front of an arc flash potential) to make the testing meaningful.

But commonly, some available noncritical small load is used. The real load, the critical load, has been switched over to the back up switchgear. Which we trust is in fine shape, since it’s last PM cycle. With on-line, constant thermal monitoring, one would know.

Due to the basic relationship of I2 x R = W, it is critical that the proper load is applied to the switchgear being tested that is the same level as your load, or tests the capabilities of your switchgear.

Notice, it’s not I x R , (which will solve for Voltage), and it’s not a direct linear relationship. The relationship for calculating power in the connection includes I2, (Current Squared) or 100Amps2 ; For this example, lets use 1 milliOhm (0.01 Ohm).

Sounds incident enough of a resistance value?
(100Amps x 100Amps) = 10,000 x the
Resistance of the connection = Wattage
10,000 x 1milliOhm (.01 Ohm) =
100 Watts
“Wattage” = the energy/heat in the connection, which you’re hoping is in fine condition and checking based on temperature.
But at 500Amps
(500Amps x 500Amps) = 250,000 x
the Resistance = Wattage
250,000 x 1milliOhm (.01 Ohm) =
2,500 Watts
Now at 2,000 Amps
(2,000Amps x 2,000Amps) =
4,000,000 x the Resistance = Wattage
4,000,000 x 1milliOhm (.01 Ohm) =
40,000 Watts!

So one can see the important role the load current plays in actually finding, detecting a bad joint.

Resistance of the connection, which one may think is even subtler, plays a surprising role in the heat within the connection. If you would like more information on the role relatively “small” resistant numbers play in failures, please contact us for “The Mighty Milliwatt” white paper; bkern@psc-exertherm.com .

Back to the Hazard… I mean the “subjects” at hand and a solution.
1) Labor intensive, mechanical intrusion that can be the catalyst for an arc flash event.
2) Having the proper loads for testing the switchgear. These two basic issues share a simple common solution… Monitor the health of the switchgear while it’s closed and under normal operation with the real load! It’s that simple.

Being able to monitor the switchgear while it’s closed eliminates the mechanical intrusion that can cause arc flash events.

Keeping the switchgear cabinets assembled and closed does not expose the electrician to the possibility of an arc flash hazard.

To achieve system level loading, some facilities rent temporary AC load banks. As discussed above, this is much more accurate than using a small token load. To use these loads banks, proper installation is important, as well as being to accommodate and cool these large load banks. Improper installation and connections have resulted in outages and damaged switchgear. So using the real load, under normal every day operation, under normal conditions is just not an intrusive act and is a natural solution that will mitigate many risks as well as arc flash.

IR is an accepted means of detecting the heat energy from losses that emanated from the resistance of a connection. Simply install IR sensors within the switchgear to monitor the connections 24/7.

No needed for an IR sensor at every junction or bolted connection. The bus bars within the switchgear conduct heat very well.

The heat generated by the resistance of the contacts within a breaker will conduct out onto the bus bar very well. So even the condition of a breaker’s contacts can be evaluated before “opening” it. Again, as with the bus bar connections, this constant thermal monitoring while the switchgear is closed gives a solid indication of the health or potential hazard the breaker poses before a worker interacts with it.

Such on-line constant thermal monitoring (24, 7, 365) not only provides real time data, and alarming, but also records monitored temperatures. So now we can evaluate the condition of the breaker, bus bars, and any critical current carrying components while under normal or heavy loadings times.

If evaluation of the data, from either monthly reviews, or from an alarm, indicate maintenance is needed, the switchgear can be de-energized, repairs performed, reassembled, closed up and re-energized. So as long as proper procedure is followed, the personnel working on the switchgear are never put in harms way of an arc flash event during the maintenance. The only time the switchgear is re-energized is after it’s fully assembled and buttoned up as it should be.

With on-line constant thermal monitoring, the engineer can immediately, safely check the condition of the repair while in front of a PC monitor, not while standing in front of a potential arc flash hazard. Will future upgrades be detrimental to aging equipment?

The health of the electrical infrastructure can be evaluated after increases in the load have occurred, such as the addition of new servers. Simply compare the recorded data temperature data from the days before and after the upgrades.

With on-line constant thermal monitoring we now can sharpen the image in our crystal ball of the health and life of the switchgear by providing true PdM while mitigating risks of arc flash.

About the Author:
Bob Kern is the ExerTherm Product Manager, with Power Service Concepts. Power Service Concepts is the authorized U.S. distributor for ExerTherm.