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Industrial Power System Configuration – Main Tie Main

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Industrial power systems can become very complex, and the related processes need a reliable source of electricity to keep the process running, both for economic and safety reasons. There are many different kinds of distribution systems. This article will only focus on one of them: the Main-Tie-Main system configuration.

Industrial Power System Main-Tie-Main Configuration

Main-Tie-Main, also formally referred to as a “secondary selective system” consists of two independent circuits connected together at the load buses by a tie breaker. See the figure below for details.

A simple main-tie-main system configuration
A simple main-tie-main system configuration

Reliability

The biggest advantage that a for an industrial power system that a main-tie-main configuration has over other system configurations is reliability. Usually, the tie breaker is normally open and the system acts as two independent circuits supplied by two independent sources. For example, we will assume that there is a fault on Source 2. This fault trips CB2, cutting off all power to Load 2. Immediately after power is removed from Load 2, the Tie Breaker closes. Source 1 is now providing power to both Load 1 and Load 2, and the system is able to perform its normal functions until the fault at Source 2 is repaired. When normal power is restored and CB2 is closed, the Tie breaker opens and the system resumes normal operation. Operation is only interrupted for a very brief moment, if it is interrupted at all!
The Main-Tie-Main configuration is also good for maintenance for this very same reason: you can open CB2 and perform repairs upstream from Load 2 de-energized, while still supplying Load 2 with power.

Transformer Sizing

One characteristic of the industrial power system main-tie-main configuration is that both transformers must be sized to appropriately handle the load of both buses. In our example, we will assume that both Load 1 and Load 2 are the same size. Transformer 1 and Transformer 2 must each be sized so that in normal operation they are only loaded to 50%. This way when the tie breaker closes, the transformer that is now supplying both loads doesn’t become overloaded and blow up in your face. The downside to main-tie-main is that the added reliability inherently costs more due to the system requiring larger transformers than a system that would not tie both circuits together.

power transformer
Unfortunately, reliability costs money, and you’ll need to oversize your transformers to add reliability to your system in a main-tie-main configuration.

You can also reduce strain on each individual transformer when it is supplying both loads by adding external cooling to the transformers (like a fan cooling system). You can also simply accept that the transformers will have a reduced life in the event of a fault on a transformer or source.
You will not have just two loads in every case. In systems with multiple loads supplied by a single transformer, transformer size (and therefore cost) can be reduced by designing the system so that only essential operational loads (such as emergency lighting) are supplied power when the tie breaker closes.

Summary

For industrial power systems, a main-tie-main configuration is an extremely reliable power system distribution model, able to maintain power during a fault with little to no interruption. Unfortunately, this added reliability has a cost, whether in the form of larger transformers, extra cooling systems, or shorter transformer lifespans. These factors should all be considered when designing an industrial power system with main-tie-main in mind.
As always, thanks for reading! If you like this post but want some actionable advice, tips and information, check out our newsletter. You can sign up here or below. By signing up you get a free report on what an Electrical Safety Program is, and how to go about building one at your facility.
 

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Arc Flash Mitigation techniques – Administrative

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Hands of Couple Looking at Blueprints
The second to last line of defence with arc flash mitigation, according to the Hierarchy of Hazard Mitigation, is administrative controls. Administrative controls are implemented after a facility is designed and built, and equipment has been purchased and installed. What the administrative controls really boil down to is your electrical safety program. I’ll break down what that is below.

Electrical Safety Program

An electrical safety program is a list of rules and procedures that must be followed by employees working with electrical equipment at your facility. You may already have a safety program in place, but it is important that you have a safety program that is specific to working on energized equipment. I’ve already written a series of articles to help you get started in putting together your own electrical safety program. I’ll highlight a few key parts here with arc flash mitigation in mind.

Standards

There are a few major standards that need to be followed when working with energized equipment. The odds are high that you will need to adhere to one of CSA Z462 (if you’re in Canada), NFPA 70E (if you’re in the United States), or EN 50110 (if you’re just about anywhere in Europe). Broader organizations that produce standards include OSHA, IEC, IEEE, and ANSI. This is in addition to any standards specific to your location, including provincial and state regulations. These standards will have specific guidelines for performing work on electrical equipment (both energized and de-energized), covering everything from work practices to label and PPE selection. All workers should be familiar with the standards. At the very least, the standards should be readily available to read and should be reviewed prior to any job on electrical equipment.

Job Planning/Maintenance Scheduling

Plan your work before working your plan

Speaking of jobs, another administrative method for arc flash mitigation is job planning meetings. These should take place before a job dealing with energized equipment begins. In general, you want to make sure to plan your jobs and schedule maintenance around the critical operations of the facility. Proper planning can include work procedures such as Lock-Out-Tag-Out. You always want to try to schedule your work so that it can be done when equipment is de-energized. An arc flash cannot occur if there is no electricity. Proper scheduling also has the double whammy effect of ensuring that maintenance on one system does not impact the operation of another system. This leads to higher operational uptime for your facility.
By signing up for our newsletter, you’ll get access to a free Electrical Safety Program planning report, which covers Job Planning in greater detail.

Personnel Training

A major cause of electrical accidents, arc flash being no exception, is untrained workers wearing insufficient PPE. Well trained, well informed workers will be able to properly identify arc flash hazards and assess the risks, thereby reducing the likelihood of an arc flash incident occuring. Job specific training is mandatory before performing any new job. If the job is performed infrequently, establish a set period of time that can pass before it becomes necessary for an employee to repeat the training for that particular job. If it has been at least a year since an employee has performed a task, that employee must be trained again. Something to stress here is diligence: complacency with work can be just much of a hazard as anything else. Everyone should always be up to date on their training for any job.
Another thing that workers should be aware of is clear communication in the workplace. You can find our article on clear communication here.

Closing

Standards, scheduling, and training are just some administrative controls you can implement for arc flash mitigation. As always, thanks for reading!
If you like this post but want some actionable advice, tips and information, check out our newsletter. You can sign up here or below.
By signing up you get a free report on what an Electrical Safety Program is, and how to go about building one at your facility.

Arc Flash Mitigation Techniques – Engineering

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arc flash engineering controls featured imageIf you can’t eliminate the hazard completely, and you can’t substitute the problem areas of your system with more appropriate methods, then engineering controls are your next best bet for arc flash hazard mitigation. There are many ways that engineering design can mitigate arc flash. You could even argue that every method of arc flash mitigation falls under the category of engineering controls. Today I’ll talk about some of the big ones.

Protection Coordination Study

To begin the process of arc flash mitigation using engineering controls, the first thing to do is a protection coordination study. A protection coordination study is an essential part of arc flash mitigation because it lets you know how your protective devices will react to a fault. The most important part of the protection coordination study tells you how long a fault will be present before a device is tripped: arc flash incident energy is reduced by reducing the amount of time the fault is active. The protection coordination study will allow you to determine whether or not you need to change the the settings of your circuit breakers and relays. This is assuming that your circuit breakers and relays are digital and adjustable. If they are not, you might want to look at substituting them for newer parts.

Virtual Main System

I was going to try to explain a virtual main system, but the folks at Schneider-Electric have summarized it much better than I can, so I’ll just quote them:
“A virtual Main consists of a digital relay and CT’s added to the LV side of a substation transformer and wired to a Fault Breaking overcurrent device (Vacuum C/B) with the ability to use a “Maintenance Switch” to set a lower fault level trip or to use Short time Zone interlocking with the Secondary feeders to allow for faster tripping of the “virtual” main. “
All they’re really saying here is that you have current transformers on the low voltage side of your transformer, and when the CT’s recognize a fault they are wired to trip a breaker on the high voltage side of the transformer, protecting the entire circuit. You can find more information about maintenance switches and zone interlocking in our article on substitution.

Optical Relaying

A conventional mechanical relay might not be enough to protect against arc flash

Optical relays are a relatively new concept. Instead of tripping solely based on overcurrent, optical relays sense the light generated by an arc fault and use that as another trigger for tripping. An optical relay requires both overcurrent and the light generated by an arc fault to trip: the relay will normally not trip if no overcurrent is detected (but it will trip if there is high overcurrent but not light). These optical relays can have tripping times as low as 2.5 ms!

System Grounding

High resistance grounding limits single-line-to-ground fault currents to very low levels. These low current levels translate into low incident energy levels and therefore reduce the risk of an arc flash hazard occurring. From the IEEE Red Book (Std 141-1993, pg 367), “There is no arc flash hazard, as there is with a solidly grounded system, since the fault current is limited to approximately 5 A). You have to be careful if using this method of arc flash mitigation: high resistance grounding doesn’t mitigate arc flash in the case of line-to-line of 3-phase-to-ground faults. Some more information about how high resistance grounding pertains to arc flash can be found here.

Location Design

Where you put your equipment is important.

Location, location, location. The location of equipment is very important. An arc flash causes rapid increases in temperature and as a result, rapid expansion of the materials that the electricity is flowing through. The explosive action of metals being vaporized and rapidly expanding is called an arc blast. Any equipment that has a high risk for arc flash should be placed in your facility in such a way that the effects of the arc flash incident are not magnified by location. For example, if an arc flash takes place in a small area, there is much less space for the energy to dissipate, and the effects of the arc flash will be concentrated. This is something that is more difficult to rectify after a facility has already been constructed, but is something to be considered during the initial design stages of any new building used to house electrical equipment.
 
So there you have it, five different techniques to consider when looking at arc flash mitigation from an engineering controls: protection coordination studies, virtual mains, optical relays, high resistance grounding, and location. This is by no means an exhaustive list, and an important thing to remember is that your arc flash mitigation techniques will be unique to your facility by necessity. As always, thanks for reading!
If you like this post but want some actionable advice, tips and information, check out ournewsletter. You can sign up here or below.
By signing up you get a free report on what an Electrical Safety Program is, and how to go about building one at your facility.

Arc Flash Mitigation Techniques – Substitution

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If you can’t eliminate an arc flash hazard, the next best arc flash mitigation technique is substitution. Unfortunately there are not a lot of ways to perform substitution to mitigate arc flash: electrical energy cannot simply be replaced by some other form of energy to provide power to equipment.

Substitution of Electrical Equipment

You can, however, replace equipment. After performing an arc flash study, you may discover that one of the reasons for high incident energy is that your equipment is old and out of date. If equipment is out of date, it must be replaced: replacement equipment could be selected for a number of reasons, including:

How can you improve the safety of your system?

Zone Selective Interlocking

Another way to reduce arc flash incident energy is by taking advantage of Zone Selective Interlocking (ZSI). ZSI allows your breakers to communicate with each other to provide the fastest tripping time possible. You can read more about ZSI here. An important thing to note is that in order to take full advantage of ZSI, you need to make sure your devices are coordinated.

Maintenance Switch

You can also mitigate arc flash hazards by adding an arc flash reduction maintenance switch to your system. The maintenance switch reduces incident energy levels on equipment downstream from the maintenance switch. The maintenance switch has its own analog circuit which is designed to trip faster than digital circuit breakers. More information can be found here.
To summarize, there isn’t a whole lot of substitution that can be done when dealing with electrical power systems. However, you can still replace out of date equipment with arc resistant ones. You can also add equipment that can perform zone selective interlocking, and can add equipment with a maintenance switch to your system.
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Arc Flash Mitigation techniques – Elimination

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Arc flash mitigation is important: to ensure the safety of a worker, every possible step must be taken to mitigate the arc flash hazard. Following the generally accepted hierarchy of controls, Elimination is the best way to mitigate an arc flash hazard.

De-Energize Equipment

When thinking about arc flash mitigation, this is the most straightforward way to eliminate an arc flash hazard. Simply de-energize all equipment that is being worked on. Without the possibility for current to flow, an arc flash incident cannot occur. All workplace electrical safety programs should stress that de-energizing equipment before work (such as maintenance) is a necessity. In order to ensure that you your equipment remains de-energized throughout the duration of the work and is not accidentally energized, you need to have appropriate safe working practices in place.

Lock-Out, Tag-Out

Examples of lock-out, tag-out safety controls that help with arc flash mitigation

Lock-Out, Tag-Out (referred to as LOCO from here on out) is an excellent form of arc flash mitigation used to eliminate the risk of an arc flash incident occurring by ensuring that equipment remains de-energized while it is being worked on. LOCO requires that employees lock and tag the device being worked on so that it is impossible to accidentally turn the device on. The device can only be energized when all employees have removed their lock and tag and have confirmed that the device is ready to be energized again. This procedure does not work when it is necessary to perform work on energized equipment.

Remote Work

Arc flash mitigation would be much better if this work was being done remotely

Sometimes equipment cannot be de-energized for work. When this happens, an arc flash incident cannot be eliminated entirely. With arc flash mitigation, what is most important is eliminating the risk of personnel injury caused by an arc flash incident. In this case it may be necessary to do work remotely. A few examples include:

  • Sensors can be added to equipment to monitor voltage, current, and temperature so that these readings can be observed on a computer instead of being observed by hand.
  • Switchgear can be closed remotely.
  • A remote racking system for racking circuit breakers will remove personnel from the arc flash boundary when inserting or removing breakers.

 
The best way to execute arc flash mitigation is to eliminate the hazard all together. You can do this by making sure equipment is de-energized for work using procedures such as LOCO. When it is not possible to de-energize, you can perform as much work as possible remotely using specialized equipment like sensors or remote racking systems.
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