Protecting switch contacts when energizing DC contactors

Protecting switch contacts when energizing DC contactors

Companies often call on Matrix to help solve problems that arise during new installations, normal operations, and/or upgrades. This small case study examines how Matrix engineers helped an energy company investigate the cause of catastrophic damage to switch contacts encountered during an upgrade of their coke and by-products facility. Matrix found the root of the problem, weighed potential options, and ultimately provided a successful solution.


Prior to engaging Matrix, the company had ordered a control systems upgrade to the first of two Pusher Machines, which involved adding a PLC to improve operational safety and control of the Leveler Bar.

Pusher Machine
Figure 1 – Pusher Machine

Upon startup and commissioning, several of the 24VDC interposing relays suffered immediate catastrophic damage (see Figure 2). Matrix provided investigative and engineering services to discover the source of the problem and come up with a solution for the failing interposing relays.

One of several destroyed interposing relays following one-time use!
Figure 2 – One of several destroyed interposing relays following one-time use!

Finding the Cause of the Damage

As part of its investigation, Matrix reviewed the schematics for the new and existing Leveler Bar controls and also gathered information on its components. Figures 3-6 illustrate the changes in schematics resulting from the facility’s control systems upgrade.

New PLC digital relay output module
Figure 3 – New PLC digital relay output module
Leveler Bar control circuit showing where interposing relay contacts were inserted
Figure 4 – Leveler Bar control circuit showing where interposing relay contacts were inserted
250VDC contactor 71F – NEMA size 3, 12” height
Figure 5 – 250VDC contactor 71F – NEMA size 3, 12” height
Resulting circuit
Figure 6 – Resulting circuit

A study of the installation revealed the following was occurring:

  • Energizing the DC contactor: Upon closing the interposing relay contact, the voltage across the inductive portion of the DC contactor coil (once the transient effect time has passed, calculated as: 5 x π = 5 x L/R = 333msec), will be 0VDC and the circuit’s current will be: 250V/1500Ω = 0.17A
  • De-energizing the DC contactor: Per manufacturer data, the typical “switch-off” time delay for the interposing relay contact is < 7msec. Assuming the longest time delay of 7msec, the resulting voltage across its contacts will be: ecemf = – L di/dt = –100H x 0.17A/0.007sec = -2,430VDC, far exceeding the rated maximum switching voltage of 300VDC. Note: the lower the switch-off time delay, the greater the voltage will be!

Applying a Solution

Once we found the cause of the damage, our goal was to find a way to suppress the excessive voltage and its destructive electric arc across the interposing relay contacts. We considered two options:

Option 1

Matrix first considered the most common solution: install a diode rated for the circuit across the contactor’s coil. In this scenario, when the contact opens, the current generated by the collapsing magnetic field will flow through the diode and back through the coil and continue until the voltage decays to zero.  Commonly used names for using a diode in this application are: kickback diode, flyback diode, and snubber diode.

Option with diode installed
Figure 7 – Option with diode installed

Option 2

The second option was to find a manufactured product for the 250VDC contactor. Fortunately, we were able to special order an arc suppressor from the same company that had manufactured the large contactor. While we could have built an arc suppressor on site, ordering from the same manufacturer ended up being the preferred solution because it was the ideal match for the contactor and of rugged construction. The arc suppressors suppressed the excessive voltage, eliminating the destructive electrical arc that was causing damage to the relays.

Arc suppressor in place
Figure 8 – Arc suppressor in place
Figure 9 – Panelboard showing Leveler Bar contactors and newly installed arc suppressors


Figure 10 – Switch ContactsOur Experts Do It Again

When faced with a tough problem, yet another company turned to Matrix for a solution. Matrix engineers applied their vast experience and expertise of electrical physics, applications, and schematic analysis to discover the root of the problem. Installation of the arc suppressors across all contactors controlled by interposing relays proved to be the answer to protect the new control circuit and its components.

This is but one of several success stories where Matrix used its technical resources to provide a cost effective and permanent solution!

Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our multi discipline engineering consultant services, contact Brian Haury, PE, Discipline Manager.

© Matrix Technologies, Inc.

Sustainable Lighting Upgrades – Opportunities for Improvement

Sustainable Lighting Upgrades – Opportunities for Improvement

Lighting System Upgrades: The Switch to LED

As companies steer their operations toward more efficient operating models and strive to hit corporate sustainability targets, every opportunity for cost and energy reduction needs to be examined. A frequent target for operating cost reduction is facility lighting system upgrades.

Lighting systems within a facility, and exterior to a facility, are often a very visible reminder to plant management of the overall sustainability efforts of the facility personnel. Everyone is affected by the lighting systems of a facility; therefore, it is critical that the lighting systems be effective, efficient, and well thought out. And though cost is certainly a consideration, it is important to balance costs with the need to ensure each location in a facility is properly lit for optimal functionality. With the continued improvements in LED lighting, most facilities are switching to LED lighting; however, if this transition is not done correctly, the result can be a poorly functioning system that is unhelpful to employees and management – and ultimately a waste of money.

A Common Pitfall—and a Shift in Thinking

A common mistake—and likely the worst mistake—in lighting improvement projects is the “one-for-one” replacement project. In this type of project, fixtures are replaced one-for-one from high pressure sodium (HPS), metal halide (MH) or fluorescent (FL) with an equivalent LED fixture. While this approach can result in money savings due to the lower power requirements of the LED fixtures, often the illumination levels are not properly matched. These ill-matched solutions often result in over-lit facilities which can “wow” people but not serve the lighting solution well.

A better approach is to consider the lighting upgrade project as an opportunity to improve the facility lighting, not just replace fixtures. While replacing HPS or MH with LEDs will almost certainly reduce operation costs, some replacements of T8 FL fixtures do not produce significant energy savings. By understanding what is being replaced and why there is a need for replacement, facility managers can move away from the replacement mentality and view the project as a facility improvement.

So how does someone accomplish this shift in thinking? It starts with understanding what parts of the facility need illuminated, how much illumination is needed, when it is needed, and how it can and should be controlled.

The Right Questions to Ask

There are multiple questions to consider when determining the optimal approach to a lighting system upgrade:

As a facility, has a lighting survey been completed to determine where illumination is needed, and how much?

Have existing illumination levels be quantified with a light meter to determine the goals of the lighting system upgrade? 

Have IES guidelines, Life Safety Codes, insurance requirements, etc. been reviewed?

Have the exterior lighting installations been reviewed to look for actual requirements, including dark sky conformance?

Are there opportunities for daylight harvesting, often the least expensive lighting solution?

Have improved color temperature lighting affects been factored into fixture replacement?

What is the best improvement for lighting controls?

The answers to these questions can have profound impacts on the design of the improved lighting system. A lighting survey helps pinpoint actual, rather than perceived, needs, and examining the current illumination levels provides much needed guidance on the goals for the upgrade. Examining existing codes and guidelines ensures that any approach is meeting industry standards, and considering the environmental factors of daytime and nighttime use ensures that the facility will not only be optimally useful during one or the other. Each of these questions demonstrates the need to move beyond one-for-one replacement thinking and consider a lighting project as an important facility upgrade, ultimately providing the best results.

In most cases, these questions lead facility managers to make the change to LED light fixtures, as they offer many improvements over older lighting systems. This change can be cost-effective, as often the dollars per lumen of output for LEDs are a lower cost than MH or HPS. LEDs also come in almost countless lighting distribution patterns, giving more design options. For example, LED replacements offer significant light quality improvements over the orange HPS lights that are notorious for their power light quality. LEDs also have much lower power requirements, often with universal voltage supply, allowing for easier power distribution and freeing up space in electrical panels.

In addition to all of these improvements, LED lighting systems are void of some of the control limitations on older lighting systems, such as re-strike times. No longer is it “better” to just leave a fixture “on” to prevent re-strike delays, as LED fixtures can be switched on/off frequently with virtually no delays. Occupancy sensors, illumination level sensors, network controls, timing systems, and more can all be seamlessly integrated into newer LED lighting systems.

Rely on Matrix for Your Lighting Upgrade Needs

Upgrading a lighting system involves many considerations, making what may seem like a simple one-for-one replacement job into a more complicated facility upgrade. Matrix Technologies can examine your existing lighting system, help you ask the right questions, and determine the optimal lighting solution for your facilities. As an outside resource, we can bring a fresh set of eyes to your facility, pull on our experience from other projects, and apply our expertise to finding the optimal solution for you.

Matrix Technologies is one of the largest independent process design, industrial automation, engineering, and manufacturing operations management companies in North America. To learn more about our sustainability and multidiscipline engineering, contact Doug Medley, PE, Project Technical Manager 2, Capital Project Planning.

© Matrix Technologies, Inc.