Electrical safety and arc flash mitigation fundamentals, part 2: Reducing arc flash hazards

Arc flash and electrical safety insights

• Arc flash hazards: An arc flash is a dangerous energy release in electrical equipment, causing severe burns, injuries, and equipment damage. It’s triggered by faults like short circuits, loose connections, or insulation failure.
• Safety hierarchy: Effective arc flash mitigation follows a control hierarchy: eliminate hazards first, use engineering controls, then administrative controls and finally, personal protective equipment (PPE) as a last resort.
• Mitigation techniques: Key methods to reduce arc flash hazards include using reduced instantaneous settings and virtual main concepts. These methods significantly lower incident energy, enhancing safety without extensive equipment upgrades.

Arc flash and electrical safety are critical aspects in a manufacturing facility. Knowing and understanding the fundamentals often mean the difference between life and death. Companies and workers need to stay on top of the electrical codes so they can better protect themselves from a potential accident.

In a manufacturing facility, an arc flash is a dangerous release of energy caused by an electrical fault that occurs in equipment such as circuit breakers, switchgear and other electrical components. This can happen during normal operation of the equipment, as well as during maintenance or repair. The intense heat and bright light generated by an arc flash can cause severe burns and other injuries to workers, as well as damage to equipment and facilities.

An arc flash event can be triggered by several factors such as human error, equipment failure, corrosion, dust, moisture and others. Such an event can happen in several ways, such as a short circuit, a ground fault, a loose connection or a failure of insulation.

Lanny Floyd, partner and principal consultant, Electrical Safety Group and Bibek Karki, power systems electrical engineer for IPS, discuss arc flash and electrical safety and what engineers need to know from the June 20, 2024, webcast: “Electrical safety: Know the fundamentals of arc flash mitigation.” This has been edited for clarity.

Bibek Karki: Both of us have worked a lot on this industry and we always like talking about arc flash hazards, mitigating hazards so we can keep our workers from the harm’s way. So I really like this hierarchy of control pyramid a lot because like Lanny said, I feel like most of the times when we talk about hazard control, we always think thinks in terms of PPE.

Everybody says, “Oh, here’s your flash suit, here are gloves, go to work.” But I feel like this pyramid highlights are outdated. We should be focusing a lot more on eliminating a hazard as much as possible. As Lanny said, the higher order controls, then substitution engineering control, and then you have admin control and PPE.

I always like to say that PPE should be a last resort. Should we use PPE? Absolutely we should, but after the fact that we have tried and done all the other methods of control. So today we’ll talk a little bit about the calculation side of our press hazard. We’ll talk incident energy, how incident energy can be calculated, can be quantified. And we will talk a little bit about a couple of methods that we have tried and used and provides a great deal of pretty much educating our users on these methods and arc flash hazard mitigation. So arc flash hazard, as Lanny said, arc flash hazard includes flash and a blast. Basically whenever a fault current or short circuit even happens, you have a arc flash and a blast hazard. Like I mentioned a little bit ago, it can be calculated and quantified by incident energy.

We really like to quantify our hazards so we know what hazard is present and then we can mitigate that. Incident energy is basically calculated in working distance. Working distance is something I will be talking a lot about because that to me is one of the most important factor in calculating incident energy and also is a big factor in mitigating arc flash hazard. Incident energy, so basically internet energy depends on three different factors. One of them is available fault current, another is a protective device fault current time. We call really trip time, circuit breaker trip time, fuse clearing time, any and everything, that includes the device clearing time. And the most important thing to me is the working distance. Again, incident energy is directly proportional to available fault current. For example, typically your arc flash hazard or incident energy can be higher right at the service entrances for most facilities because that’s where utility provides tremendous amount of energy directly proportional to the fault clearing time.

What I mean by that is the lower your fault clearing time, the lower your hazard. For example, modern relays, microprocessor relays, electronic trip circuit breakers, they typically have a lot faster clearing time compared to your thermal magnetic breaker, motor circuit protector or older model circuit breakers with a discriminatory type of trip units. And again, most important to me is inversely proportional to a square of working distance. Basically, if you move two feet away from a potential electrical equipment which could have any type of outlet hazard on it, your hazard decreases by four times. This is one of the most important factor on the hierarchy of control on the lower side of things. So on today’s presentation, we’ll be talking about fault clearing time and we’ll be talking about working distance.

We’ll be focusing on elimination and substitution/engineering control. So basically, with elimination we’ll be talking about there are three different methods that has been used by a lot of vendors, a lot of engineering firms, a lot of people who do arc flash studies, but today we’ll be focusing on reducing instantaneous 50P settings and also using concept of virtual main and there’s a case study on that as well. Another side of that pyramid that we’re going to be talking about is substitution or engineering control. I know these are a little different methods, however I like to use them together, because the two methods that I’ll be talking about include almost half of each these.

Example of arc flash hazard mitigation using reduced instantaneous 50 settings. Courtesy: Plant Engineering

This is probably one of the favorite methods that I like to talk about and use a lot about reducing arc flash hazard. The reason this is really popular is because this can be used all across the board. You can apply this to a 208 system, 480 volt, medium volt is relaying anything. This is probably one of the widely used method. For medium voltage relays, this is typically implemented with multiple group settings. On your normal operation you have your normal instantaneous setting, because if you use your reduced instantaneous setting on normal operation, you run into issues of miscoordination, tripping, inertia stripping on the load, things like that. So for medium voltage setting, this is implemented using multiple groups. For a low voltage setting, you can see different vendors call this method different names.

How does this work? On the time current curve (TCC) that I have created on the right-hand side, you see there’s basically a little one line diagram on the bottom left corner. That’s a system that I’ve modeled and then you will see there’s a blue TCC and there’s a red TCC. The blue TCC is basically that is your relay curve. The red TCC is basically your relay curve for your normal operation. On a power system, you perform a study and then you set your device according to the study. That would be a red curve. Now the way this method works is we create a blue curve. Now once you perform arc flash has an analysis on a system, you know available fault current on that particular bus. So as long as you set your protective device right underneath that fault current, that trips a lot faster reducing your hazard.

So that’s the principle that this method works on. Basically, the blue curve is what we do. Once the incident energy calculations have been performed, we are creating this blue curve which is a reduced 50P settings. Set 50P below the available fault current and then you can use any type of control mechanism. For example, control switch, relay logic, push button to alternate the group settings. The key here is you do not want to leave your reduced 50P setting on during normal operation. You typically use this while performing any maintenance or any type of interaction with the electrical equipment. Again, like I said, this could be implemented on any power system with relays, circuit breaker, anything. Another important thing to mention about this topic is that National Electric Code now requires you to have a reduced RELT system on any service entrance, which is bigger than 1200 A.

This is one of the most important method that we’ve used and found very effective. Okay, another method that works great. Again, this can work on a new or a retrofitted gear, is arc flash mitigation using concept of virtual main. In a typical true main utilizes concept of differential, so basically the way differential works is your relays trip very fast. We’re talking a couple of cycles, two to three cycles. The fault clearing time is very low, which reduces your arc flash hazard or incident energy. Now, the only downside to a typical virtual main is that involves significant hardware upgrade, your switch gear would have to be overhauled, you have to get a new switch gear, you have to get new different current transformers, relays, so it can be very prohibitive.

In order to kind of bridge that gap of the capital cost, another concept that could be used is the concept of virtual main. We are basically utilizing the existing medium voltage circuit breakers and relays so it can be a cost-effective process. All we are doing is installing current transformer on the secondary of transformer and we are programming the 50P on the upstream relay. I have a case study where I’ll explain a little bit more on conceptual on how this method works.

As you can see, there is a little impedance diagram that I’ve created which basically has a utility source, has a protective relay, minimal voltage to circuit breaker immediate to the minimal circuit breaker is as a transformer downstream and then there is a low voltage switch gear with copper breakers. So there is no physical main, so 1240 to 480 system with no physical main 2,000 KV transformers.

The incident area was very high on the 480 volt side of switch gear. Again, lower voltage, higher current, which leads to higher available fault current, higher incident energy. Predictive relay on the primary of transformer had over current capabilities like we talked in the previous slide about 50, 51 function. The project scope was basically installed current transformer on the secondary of transformer. Again, try to minimize the switch gear overhaul so we can make it cost-effective solution. The zone of protection expanded for this relay in primary of a transformer to the secondary now. Those current transformer that was installed on the secondary of the transformer were wired back to the relay on the primary side of transformer.

The predictive relay was programmed to reduce 50P settings, like I talked about the first method on a couple of slides before. Using this process we ran a simulation and any senior energy was reduced from 137 calories per centimeter squared to 6.85 calories per centimeter squared. Again, with not a lot of significant switch care overhaul, as you can see, the incident energy was dramatically reduced from 130 to almost single digit. Something like this could be implemented which works in a great deal of reducing arc flash hazard.

Those two were the methods that would eliminate hazard. Now as Lanny mentioned on that pyramid, if you cannot eliminate hazard then you go to the next process and try to mitigate them somehow, some way. These two methods that I’ll be talking about next are basically part of engineering control or substitution. First one is a access hazard mitigation using delay timer and push buttons. The way this works is most over current protective relays have front panel push buttons. Now, if the facility has a very older model relays, then they would have to upgrade those relays to the new relays which have the push button capabilities. Again, this is again one of the cost-effective solution, because you are upgrading a relay. However, most vendors provide these features without any additional cost.

The way this works is select a relay with front panel push buttons. Most really have a very simple logic that we could utilize to program delays. Most control switches come with a timer. What you do is you typically program a 10 to 12, 10 or 20 second delay on a trip and close functions. So typically tripping a circuit breaker and closing a circuit breaker are one of the most important thing that we do on a power system. And also one of the most hazardous tasks if not done properly or if someone without a training does it. What we’re doing is we are programming it as delay on those functions. The way this works is you push a button and there’s a little light that’s just blinking. Basically, let’s say you have program a 20 second delay. Once you push a button, you have 20 second to get out of that switch gear. What we’re essentially doing is we are increasing a working distance while operating equipment.

And again, like I mentioned in my first slide, once you increase your working distance, your arc flash incident energy decreases dramatically. This little delay provides the opportunity for a user to get out of the harm’s way and essentially you’re mitigating, you’re using this as a substitution or engineering control method. Another method on something like that is remote operation using relay or PLC logic. Basically, the whole premise of this is you are performing all the operations remotely. So there’s no operator in the switch care room. All you are doing is like I mentioned a previous side, you’re increasing a working distance here.

Again, programming a relay logic. Basically, you would program your relay logic remote bid in circuit breaker operations, which allows the operator to open and trip the circuit breaker from control room. This can be done on a programmable logic controller (PLC) and distributed control system (DCS), as well. So pretty much, it’s the same principle. The whole thing is we are performing all the operations remotely, so we are using distance as our frame on these processes. Again, with a lot of these methods, there’s been so many changes in technology and also research and development, so I’m sure these would increase and help our workers to become safe.

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