SPD & MCB Coordination: The Hidden Cause of Downtime & Failures

When an engineer selects an SPD for their system to protect the devices from overvoltage from lightning strikes, they are selecting one of the many critical components needed for perfect functionality. An equally important consideration when considering the SPDs selected to provide overall protection is whether or not they are properly coupled to the Miniature Circuit Breakers (MCBs). If the SPDs are not properly sized and each is still adequately chosen to cooperate with each other for the protection of the installation, this inadequately coordinated equipment can account for 40 percent of issues caused by surges.

Whether it's downtime, equipment failures, or compliance issues, this lack of proper SPD/MCB coupling can create significant issues for a facility or an industry. Understanding the basic purposes of Surge Protective Devices (SPDs)and Miniature Circuit Breakers (MCBs), including their inherent capabilities and limitations, is necessary for successfully coordinating SPDs and MCBs to help prevent your electrical systems from experiencing downtime or other types of failure. You will also have to coordinate SPDs and MCBs in accordance with the requirements of any industry standards imposed by Statutory Authorities (SA) (government).

It is expected that an SPD will only trip if a fault or a short occurs within the SPD or the electrical circuit/equipment connected to the SPD. A surge protective device (SPD) that is improperly coordinated with an MCB can lead to both nuisance tripping as well as the potential loss of property due to damage caused by the SPD.

Money-Draining Factors Associated with Mismatched SPDs and MCBs

SPDs and MCBs that have been mis-coordinated will create numerous incidents of frivolous and expensive activity, due to three reasons:

1. Nuisance Trip Events

Upon experiencing a harmless surge, an improperly selected MCB may erroneously trip, terminating the operation of equipment that is critical to an organization, i.e., production facilities, data centers, and automation equipment. The resultant downtime of equipment that has experienced nuisance tripping can eliminate revenue flow and frustrate workers.

2. Fire Risk

Old and/or defective SPDs will usually result in short circuits or an equipment malfunction; therefore, the MCB must trip immediately following the SPD fault. If the MCB is either mis-installed or the MCB selection does not meet the proper sizing, it may take the MCB too long to trip, which allows the SPD to continue to overheat — creating smoke and/or fire risks to both humans and property.

3. False Sense of Protection

You may believe that your system is protected; however, if your SPDs and MCBs are not properly coordinated, there is a strong potential that your system could experience surge damage over time. The surge damage to electronic devices, such as PLCs, inverters, and servers, will likely cause premature failure and increased replacement costs.

How to Properly Coordinate SPDs (Surge Protective Devices) and MCBs (Miniature Circuit Breakers)

Resolving SPD-MCB Coordination failures need not be difficult. The following suggestions will help to improve the reliability of your systems, while adhering to industry standards:

1. Curve is Important; use the Correct MCB Curve

Use C-Curve MCBs for SPD Types I and II: C-Curve MCBs will withstand the short-duration high currents caused by lightning and surges, yet offer some protection from the SPD failure. C-Curve breakers are less likely to trip for nuisance reasons than B-Curve breakers.

2. Check MCB Ratings

Ensure that the MCB Short-Circuit Ratings meet the SPDs Withstand Capacity or vice versa. Installing oversized breakers will cause more damage during SPD failures than properly sized breakers due to delayed tripping.

3. Proximity Rule; Keep SPDs & MCBs Close

SPDs and MCBs should be installed within a distance of 0.5 meters (measured by conductor length). This will reduce the amount of residual voltage and allow the MCB to trip quickly if the SPD fails.

Where SPD Coordination with MCBs Will Be the Most Critical

If you have very sensitive, high-value electronic equipment or assets, coordinating the SPD and MCB together is critical. Here are the 3 locations where improving your SPD/MCC coordination will have the greatest effect:

1. Charging Stations for Electric Vehicles

Although installing an electric car charging station is costly, the sensitive equipment being used to charge vehicles requires effective SPD-MCB coordination to successfully protect it. To comply with the standard of being minimally affected by surge activity, three-phase SPDs will need to be matched correctly.

2. Photovoltaic and Battery Storage Systems

With the DC side of photovoltaic systems (solar panels) and battery storage, the successful operation of these systems requires specialized DC SPDs and appropriately rated MCBs in order to avoid surges that could disrupt equipment and provide them with dependable generation of electricity.

3. Data and Automation Centers

Even tiny amounts of surge activity from HVAC equipment and power fluctuations can disrupt data, damage sensors, and cause unplanned shutdowns in data centers and industrial automation. The correct SPD and MCB coordination helps to minimize these risks.

Frequently Asked Questions Regarding SPDs and MCBs

Do all SPDs require an MCB?

According to IEC 61643-11, an MCB is included to protect any SPD. An MCB serves as the overcurrent protection device for the SPD and also provides safe disconnection in the event of an SPD failure. If your SPD does not have a dedicated MCB installed, your warranty will probably be voided, and you will most likely fail an inspection.

May I substitute my main circuit breaker with an MCB for my SPD?

Most of the time, you cannot. A Main Circuit Breaker and an SPD will not be rated the same, and the Main Circuit Breaker will probably be oversized, meaning it may take longer to trip, which can worsen the failure of an SPD.

What causes SPDs to fail?

An SPD that is subjected to frequent minor surges degrades over time. If the SPD has failed, you should replace it as soon as possible to maintain SPD protection.

Protect Your Systems by Providing Reliable, Compliant Surge Protection

Failing to properly coordinate SPDs and MCBs can result in substantial downtime, difficulties with meeting compliance regulations, and increased maintenance costs, thereby leading to protecting your operation by making sure your surge protection system establishes the most appropriate match to your individual needs.

We design IEC 61643-11 compliant SPD-MCB solutions for residential, industrial, and electric car charging applications. Let our experienced staff conduct a free coordination review to take the guesswork out of coordinating your system together. Whether you want a consultation, would like to get multiple quotations, or need product recommendations, we can help. Contact us today to ensure that your critical electronic systems are protected.