“Electricity is really just organized lightning.” George Carlin
When I think of circuit breakers, I usually think of the circuit breakers in the panel in the basement at home. They are relatively small (120 to 240 volts, 10 to 60 amps), and they’re for fairly simple circuits. They supply power to my lights, outlets, refrigerator, television, washer/dryer, and air conditioner. I can control power to different points in my house with the flip of a switch, all thanks to the circuit breakers on this panel.
In an industrial setting, there may be breakers for 5000 volt service, or even 15,000 volt service. There are even bigger breakers, but they are not what we’ll come across, even in most industrial settings. However, we are much more likely to encounter breakers for 480 volt service. These breakers aren’t there for toaster ovens. They are there for multi-horsepower pumps, compressors, and other pieces of industrial equipment.
Tell me again, why do we have breakers?
First and foremost, breakers are for safety. Think about those breakers in your basement. Normally, you only think about them for one of two reasons. Either they have tripped, in which case you curse the darkness as you descend into the basement to reset them. In this case, they’ve tripped in order to protect the circuit from high current, which can lead to circuit damage, overheating and fire. Or you flip them deliberately as an isolation device, where the breaker disconnects an electrical source from its load. This allows you to work on a circuit without being exposed to dangerous voltage.
These are both safety functions and in our homes, we take it for granted that they will work the way we expect.
Breakers also serve other functions. They can act as fault protection, where they automatically trip due to an electrical fault. In an industrial design, depending on the design, breakers may also allow a user to transfer a load from a lost source of power to an alternate power source, either manually or automatically.
When breakers work
When a breaker trips in our homes, it is either to protect against overcurrent or ground fault. The typical response is to reset them and assume that everything will be okay. When they trip again, then we try to figure out what’s wrong and address the root cause. That’s when we unplug the second hair dryer and move the electrical space heater to a different outlet, or discover that the insulation on the vacuum cleaner cord is starting to wear through.
An industrial breaker can trip for a variety of reasons. In addition to tripping on overcurrent and ground faults, breakers can trip on loss of voltage as well as on sensing an arc flash. Breakers may also have fault detection settings that coordinate with upstream protective devices in order to isolate the fault with minimum interruption to the remaining electrical system.
This breaker coordination is based on either current discrimination between upstream and downstream devices, or time discrimination between upstream and downstream devices. Time discrimination is based on the response clearing time of each breaker, so that downstream breakers trip before upstream breakers. When they do, a breaker can perform its intended function without the rest of the system going down. When they don’t, then an upstream breaker can unnecessarily take the whole system down, which may pose its own safety consequences.
Industrial breakers are also our means of lockout on electrical equipment. Lockout not only isolates energy from equipment that is being opened up for maintenance, it prevents inadvertent startup while maintenance technicians work on it.
When breakers fail
Breakers can fail in any number of ways. They can trip prematurely and they can fail to trip. Contacts can be slow to open. A tripped breaker can fail to open all contacts, fail to latch, fail to quench an arc, or fault to ground. While the failures modes vary, they all have one thing in common: they can be avoided with routine maintenance checks.
Manufacturers typically recommend a maintenance inspection of breakers at a frequency of once a year. For industrial facilities with an annual plant outage, the outage is the time to do it. Sadly, due to a lack of time and manpower, lower and medium voltage breaker often get overlooked, especially if they are fixed mounted molded case circuit breakers. This results in a likelihood of breaker failure higher than expected. A 2008 survey conducted by the InterNational Electrical Testing Association (NETA) examined the performance of 340,000 low voltage breakers that didn’t follow the maintenance schedule recommended by the manufacturer. A staggering 43% had mechanical issues affecting their operation, 23% had an issue affecting the protective device operation, and 11% failed to operate at all.
Others have shown that if a circuit breaker is not properly maintained over a five-year period, per the manufacturers’ instructions, there is a 50% probability of failure of the circuit breaker.
What does this mean?
It’s a credit to electrical designers that we simply expect power systems to work and to work safely. However, without proper routine maintenance, the probability that a circuit breaker will fail to respond properly to a demand is much higher than we assume. Likewise, the calculated incident energies can be much worse than we assume.
Breakers that are not working correctly can result in the personal protective equipment selected using the tables in NFPA 70E being inadequate. Breaker failures can also invalidate arc-flash studies due to clearing times becoming an unknown element.
The failure of breakers to provide the protection we are expecting increases the likelihood that something disastrous will happen.
What needs to be done?
In order to increase the reliability of all circuit breakers, regardless of their voltage, we need to assure that routine maintenance is performed. Fortunately, several standards and guides exist to help users with electrical maintenance. This includes NETA, IEEE Standard 902, NFPA 70B, and NFPA 70E. As a last resort, we can always turn to the manufacturer’s instructions.
For low voltage circuit breakers, NETA suggests performing a visual inspection once a month and a visual plus maintenance inspection once a year.[1]
NFPA 70B, on the other hand, suggests a longer time span between maintenance. It suggests performing inspection and maintenance for drawout breakers “every 3 years or at manufacturer’s maximum number of operations since previous maintenance, whichever occurs first,” and between every “3 – 6 years” for molded-case circuit breakers.[2]
Regardless of the standard each of us chooses to follow, developing a routine breaker maintenance schedule is critical. Without one, the validity of a plant’s arc-flash studies, calculated incident energy, and personnel PPE selection decreases, while at the same time, the likelihood of electrical devices failing to operate properly increases.
When process risk analysts take the failure of a circuit breaker into account, the failure rates we use take proper maintenance as a given. We need to make sure that it is a given.
[1] Figure 7 – Section 7.6.1 1 from ANSI/NETA MTS-2011
[2] NFPA 70B, Table K.4(d) Low Voltage Equipment, Drawout-Type Circuit Breakers: Maintenance of Equipment Subject to Long Intervals Between Shutdowns – Electrical Distribution
