Electrical: Circuit Protection, part 3: Questions and answers on electrical and circuit protection

Randall Ehret and John Yoon answer additional questions from the webcast viewers on electrical and circuit protection.

By Consulting-Specifying Engineer September 1, 2023
Understanding electrical and circuit protection requirements will enable the engineer to design the safest and most reliable electrical distribution systems for buildings. Courtesy: CFE Media and Technology

Electrical protection insights

  • The electrical engineer is responsible for many different distribution systems in buildings and it is crucial to know what is needed, how much is needed and the codes and standards designed to keep people safe.
  • Failing to understand can lead to serious injury or death if not properly adhered to, which makes electrical safety all the more important.

Randall Ehret, technical director for ESD, and John Yoon, lead electrical engineer at McGuire Engineers, answer additional questions on electrical and circuit protection in the Jun. 28, 2023, webcast: “Electrical: Circuit Protection.” This has been edited for clarity.

Consulting-Specifying Engineer: Randy, how do overcurrent protection device (OCPD) strategies vary between different building types?

Randall Ehret: It really has to do with the level of care that’s required in the building. So, if you look at a hospital or a data center, we would likely never use fuses in those locations. The reason is you have to keep spare fuses. You can’t quickly reset, and so, if there’s a nuisance trip rather than a real fault condition, you have to go around and look for new fuses to replace those. So, I would often pick breakers for those types of applications. Fuses can be used, but you have to be a little bit more careful and make sure you have a good clean supply in every closet.

Additionally, when you start looking at all the different types of breakers that are available now in the market and as it continues to grow, you need to look at, again, what is the need for uptime in your building? When you start adding the electronic trip functions onto breakers, that can become more expensive and add cost. Sometimes that cost has value to the owner, sometimes it does not. And so, when I look at a commercial building where the uptime requirements aren’t as stringent, then I’m looking at more of a molded-case breaker solution. When I’m looking at a mission-critical facility, I’m usually looking at electronic trip breakers.

Consulting-Specifying Engineer: It ties into another question that was asked. Knowing which OCPDs apply to which scenarios and when to use a specific OCPD over another. You mentioned those examples. Are there a few others that come to mind or scenarios that you’ve dealt with?

Randall Ehret: Yeah, I think another thing to think about when you’re selecting your OCPD is, we have to make sure we get the right device for the right load. So, when I look at the amperage rating, ampere rating of the device, I need to look at what that load is, and there are different criteria for motor loads than there might be for lighting or receptacle loads. And so, we have to look at what type of loads we’re feeding. That would be one impact on the OCP.

Another consideration would be, again, are we trying to selectively coordinate or not? If we’re trying to selectively coordinate, we really need to consider the LSI/LSIG breakers’ electronic trips. While, if we don’t have that requirement, we can go for a more standard breaker with the understanding that, rather than my 400-A breaker tripping, my 800-A might trip, and I may take out a whole distribution board rather than just a feeder.

John Yoon: Randy brings up a good point. When we’re talking about LSIG breakers, we’re talking about solid-state trip units, and we have the ability to shape the curve to accommodate onboard overcurrent protection requirements and to selectively coordinate between our upstream and downstream breakers.

And that’s, again, to his point, if money were no object, if we didn’t have that cost restraint, we would specify them on every project. But the question is, can the cost be borne by the project, and what other things does it impact? If we spend money there, are we shortchanging ourselves elsewhere in that distribution to provide other potentially resilient items that can improve the overall reliability of our distribution? It’s just like you spend all your money on breakers, and then you have to be marginal with other equipment. It’s a balancing act there.

Consulting-Specifying Engineer: So, John, how do you prioritize selective coordination? How do you reduce arc flash energy in a project?

John Yoon: When you’re trying to balance selective coordination and arc flash energy, we’re talking about two things. Normally, with selective coordination, we’re talking about normal operating conditions where we’re trying to respond to overcurrent conditions somewhere within our distribution system. And the analogy that I like to use is when you’re parked below a tree and there’s a storm that rolls through, if something happens to that tree, you want the smallest branch possible to fall on your car. You don’t want a big limb to fall on your car, you don’t want the entire tree to fall on your car. So, again, with selective coordination, we’re trying to limit that disturbance to the smallest portion of the distribution system. Whereas with reducing arc flash energy, we’re recognizing that we’re trying to protect an individual who potentially is working on a piece of equipment rather than a normal operating condition. This is whenever you’re doing maintenance on a piece of equipment or somebody is interacting with that piece of equipment.

So, again, to reduce arc energy, we typically want to trip a breaker or open some type of protective relaying as quickly as possible, and that sort of throws the selective coordination out the window. If you recall back to that slide, we were talking about the potential methods for achieving arc energy reduction per the code. One of them is zone-selective interlocking, but when we’re talking about reducing arc energy, we want things to operate very, very quickly. So, just saying we’re providing zone-selective interlocking doesn’t really achieve the fundamental requirement of tripping that breaker as quickly as possible to reduce the amount of energy that’s released. If something bad happens, protect the person in front of that piece of equipment.

Consulting-Specifying Engineer: Randy, can you describe some best strategies that you currently use for selective coordination, particularly as it relates to existing panels and subpanels with the replacement switchboard?

Randall Ehret: Yeah. First of all, I love your tree analogy, John. So, if we have to maintain selective coordination, you still need to do the study. So, whether it’s an existing breaker or a new breaker, you have to make it selective for the code. Sometimes that requires you to replace existing breakers. But what I like to do is after I’ve gone out into the field and I’ve looked at what’s there and I have a better understanding of how that distribution system is put together, the manufacturers all publish these really nice tables that you can go to and take a look and say, “Okay, I’ve got this much fault current available. I’ve got this breaker. What will selectively coordinate with it upstream if you know the sizes?”

And so, it’s a really easy lookup to give you a good quick feel for what’s going on and whether you’re going to be looking at a bigger effort of replacing equipment in a project or being able to simply modify it a little bit and get in there and make things selectively coordinate.

Those tables are a really good tool. They’re readily available online by every manufacturer, and it’s a good starting point. The other thing that the manufacturers have come up with, so you don’t have to go through an SKM Software or something like that to do the study, is they’ve got these very quick three-level gut checks that you can use. So, they’ve got the software online, you plug in the available fault current, the lengths of the cable, this type of thing, and select a breaker and a size, and it will tell you what will coordinate with it upstream. So, there are some good tools out there if you make use of them.

Consulting-Specifying Engineer: It does sound like it’s nice that the manufacturers are going out of their way to try and simplify the process as much as possible, particularly given that you could be dealing with equipment that, if used incorrectly, could harm somebody in the process.

Randall Ehret: Yeah. And I don’t want to make it sound like these are necessarily replacements for doing a full-blown study. What they are, are tools to use to get you started and get you over that hump so you can get your design going.

Consulting-Specifying Engineer: Randy, what types of mid-construction design changes ought to trigger the need to update a coordination study?

Randall Ehret: So, I think the first thing you need to understand to answer that question is what factors impact a study. So, when any of the factors that impact a study change or vary, then you need to go redo your study. So, that could be the feeder cable length, it could be that you got some updated information from the utility company and the available fault current at the utility source is different than what you had thought previously. I just had that happen on a project last week. It could be a request for substitution on the breaker type that comes in. It could be load changes that come in, and all of a sudden you’re changing the size of a feeder or the size of a breaker. Any and all of those things will impact your study, and at that point, you need to at least go back and reevaluate that portion of the study and make sure everything is selectively coordinated.

John Yoon: Again, oftentimes, a whole bunch of things are beyond our control, things that happen in the field, some things are requested by the client that could impact the need for that recalculation. And again, if we’re trying to boil it down, we’re trying to make the system as reliable as possible, as safe as possible, and that becomes a very strong motivation to ensure that our calculations are updated on a regular basis.

Consulting-Specifying Engineer: John, do breakers lose their IT curves after years of inaction or do they not?

John Yoon: I’m assuming the question is, does that time current curve change over time? And the answer is sort of. In most cases, when we talk about breakers, we’re talking about UL standard 489 for molded-case circuit breakers. And that standard says that a circuit breaker will operate as intended into full fault rated at least once, and beyond that, all bets are off. So, there’s that.

They should operate on a regular basis as intended for an extended period of time. But if we go back to that slide of the cross-section of a molded-case circuit breaker that’s cut open, you can see that there’s a pretty intensive amount of mechanisms within that particular circuit breaker body. There are springs, there are pivots, there are a whole bunch of other things. And one of the things that is commonly not done is maintaining circuit breakers after they’re installed.

So, the simple recommendations within NFPA 70 or NECA or something along those lines, which say that you should operate circuit breakers, exercise circuit breakers every year or so, is commonly not done. And if you have all these intricate mechanisms within a circuit breaker body that pivot and go back and forth, if they stay in a single position for an extended period of time, they will typically start binding up after a while and they won’t operate as intended. And those changes in the operations of the internal mechanism will impact what that time current curve looks like. So, you might be expecting it to operate one way, but because the circuit breaker hasn’t been maintained on a regular basis, it could react in a totally different way.

Randall Ehret: I think John covered that very well, but I think him touching on circuit breakers and their requirements for annual testing and servicing, the same thing applies to fuses and the same problems can occur. So, if you do get into a facility and the breakers haven’t been properly maintained and tested, I would certainly recommend that that be done before you start adding onto that system. So, have the owner go in there and doing the OEM, make sure they have spare parts on hand when they do it, especially with fuses or bolted pressure switches, that type of device because they will hang up and make sure you know what you’re getting into.

John Yoon: To expand on that, the mechanisms that we see in circuit breakers, there are also mechanisms within fuse switches as well. And specifically things like bolted pressure switches. And if you don’t operate these mechanisms on a regular basis, they tend to seize in place. So, if you have an overcurrent protection event where you pop a fuse and you have to replace it, but the switch hasn’t been operated for an extended period of time, you may not be able to physically de-energize that switch to turn it off so you can safely replace those fuses. And I’ve been in that situation before in a mission-critical facility. And let me tell you, it’s not fun.

Consulting-Specifying Engineer: What are the differences between relays and circuit breakers?

John Yoon: So, when we were talking about solid-state trip units, we were talking about taking those fairly simple overcurrent and protection mechanisms of thermal and magnetic and removing those and achieving those through an electronic sensing function. And basically, what we’re doing is we’re starting to creep into the realm of protective relaying. Protective relaying is more common in the medium voltage distribution world where every protective function has an ANSI number associated with it. So, for example, ANSI relay form factor number 51 is overload. ANSI form factor number 50 is an instantaneous overcurrent protection. So, what we’re doing with the solid-state trip units is taking those selective relaying functions and putting them into a separate module.

Randall Ehret: And I would add, John, that now for the medium voltage breakers I think the way to go is some of the electronic relaying so that you get all these features and functions put into one unit, and you’re able to adjust things much more efficiently and effectively. And those things can actually then service multiple breakers.