Function vs. Functional Testing

Jacob Loyd and Michael Wilson, MeggerFeatures, Spring 2024 Features

Testing plays a critical role in verifying that the protection scheme is designed to meet its intended purpose. It ensures that the wiring diagrams match the schematics, and everything works together seamlessly.


While engineers and electricians are highly skilled professionals, they are human and can make mistakes. That’s why, during the commissioning process, we meticulously search for errors, knowing full well that they can exist anywhere. It is imperative to verify cable sizes, color codes, and termination labeling in even the most carefully planned designs to ensure accuracy. Moreover, the creation of precise as-built drawings is crucial for future projects. Without proper maintenance, it becomes increasingly difficult to plan and design upgrades.


In the world of commissioning, function testing involves the manual or electrical manipulation of various components — such as relays, sensors, gauges, and contacts — to verify the presence or absence of electrical signals through the different paths of a schematic. This signal can be verified by picking up or dropping out a downstream device or by using a metering device like a multimeter to measure voltage or current.

Because each device and customer design has unique schematics, creating a standard commissioning procedure that ensures point-to-point continuity through an AC or DC circuit can be a complex and meticulous process. As a commissioning engineer gains experience with exposure to schematically testing different devices and designs, they develop an appreciation for the science and the art of commissioning. 

To illustrate, consider the DC schematic of a breaker trip circuit shown in Figure 1. The normally open OUT101 contact breaks the positive leg of the 125 VDC circuit of a specific component. The circuit breaker’s trip coil is energized by pulsing the OUT101 contact, proving that the signal is passed to a downstream terminal point or device when the contact is closed.

Pulsing the relay contact also verifies the relay contact’s ability to operate from an open to closed position and back again. Note that for simplicity, we have removed the auxiliary contacts that are usually present to provide supervision.


Functional testing is an intricate process that not only checks the relay’s output contact movement from closed to open and back again, but also verifies when the contact should operate. To elaborate, consider Figure 2. We observe that the relay’s OUT104 contact is connected in series with the 86BF breaker failure lock-out relay (LOR). During function testing, we pulse the output to test the lock-out relay’s operation or roll, followed by verifying the contact development to validate the circuits connected with the 86BF LOR. 

While function testing involves pulsing the output to confirm the lock-out relay’s operation and validating the contact development to prove the circuits associated with the 86BF LOR, functional testing takes a more comprehensive approach. In this method, we examine the circumstances that could cause OUT104 to operate, such as the trip condition that exists after 18 cycles, typical in programming breaker-failure logic.

This approach allows the trip to be initiated, monitors the secondary current magnitude, and asserts a separate contact to operate a lockout relay that trips adjacent circuit breakers, blocks auto-reclosing, and sends necessary alarms to the station relay terminal unit (RTU). However, testing the operability of the OUT104 contact alone can lead to missing the conditions that must be fulfilled and dictate when OUT104 operates.

Even if the OUT104 contact is properly wired, the wiring diagrams are accurate, and the output is pulsed successfully during commissioning, there is still a chance of missing critical details. For instance, the OUT104 definition could be flawed or dependent on some other bit of logic that might have been inadvertently brought over from a previous project or an entirely different substation.

As technology advances and protection schemes evolve, testing methodologies must adapt to keep up. Commissioning engineers must ask themselves if their current testing strategies could miss critical details. If the answer is yes, they must make changes accordingly to ensure that the system is thoroughly tested and meets the desired standards.


As commissioning engineers and test technicians, our work often involves testing various devices, alarms, and circuits in newly implemented designs. Sometimes, we are required to do these tests on short notice due to necessary isolations or in-service equipment that could potentially be affected. While this approach may not be the most efficient, it does allow us to make progress while waiting for additional circuits to become available. Additionally, it offers greater insight into terminated cables and their potential impact on existing equipment, enabling us to identify and address any issues that may arise. By doing so, we can ensure that the devices, alarms, and circuits in the newly implemented design are functioning optimally and that any potential problems are identified and resolved quickly and efficiently.

Advantages of Current Testing Methods

Engineers and technicians should test protection and control circuits as they become available to ensure that all construction and commissioning activities are well-coordinated. This approach allows them to verify the accuracy of issued-for-construction (IFC) schematics and wiring diagrams and identify any discrepancies early on. Typically, commissioning activities are driven by construction activities, and technicians tend to coordinate their testing based on the availability of electricians to install, form, and terminate cables running from panels. By commissioning circuits as they become available, engineers can ensure that the construction activities are aligned with the commissioning activities and that discrepancies are addressed immediately.

It is important to note that customers often have strict protocols regarding returning equipment to service and completing the required documentation. Since the final protection settings may not be available at the start of a project, it is crucial to test the operation of inputs and outputs, even without actual relay settings. This approach helps engineers identify possible wiring or equipment issues early on that may require replacement, which could have a lengthy lead time. In the case of modifications, these problems can even be latent issues with wiring or labeling that have gone unnoticed for years. By identifying such issues early in the project cycle, engineers can ensure they are addressed while there is still enough time to commission the equipment and avoid any delays.

Ultimately, getting an early start on highlighting or verifying drawings is critical to submitting as-built drawings on a timely basis. This approach is also essential for ensuring that newly commissioned equipment is placed into service as soon as possible. Commissioning circuits as they become available is an efficient and effective way to achieve these objectives while ensuring that all construction and commissioning activities are well-coordinated.

Disadvantages of Current Testing Methods

Testing circuits as they become available has several disadvantages compared to a more structured and coordinated approach. Multiple touches are often required, which can lead to human error and wear and tear of the contact being operated. Lifting terminations during testing can also increase the risk of human error while the time required for warning others before operating devices in the substation yard or switchgear room can be substantial. Moreover, when a trip or close coil saturates and operates, an arc develops across the contacts as it interrupts the circuit. This arcing causes pitting and degradation of the contact’s life (Figure 3). Therefore, minimizing the number of operations performed on a device, such as a circuit breaker, motor-operated disconnect, or lock-out relay, is crucial to reduce the chances of degradation.

We recommend a more structured approach to testing circuits to achieve overlap in testing procedures while minimizing the number of operations performed on a device. This will reduce the risk of human error and contact wear and tear, ultimately improving the efficiency and performance of the circuit. 

For example, if testing a protective scheme on a circuit breaker equipped with a lockout relay, the first relay operation can be verified to trip the lockout relay, which then trips the breaker. Now that the circuit between the 86 and the circuit breaker has been verified as intact and functional, we can avoid additional operations of that breaker by leaving it in the trip state and testing all further trips to the 86, helping prolong the lifecycle of the breaker.


This leads us to the next question: What are the advantages and disadvantages of functional testing? Functional testing can be a complicated process, depending on the protection schemes being tested and the affected equipment. Brownfield upgrades often involve in-service equipment and require weeks, if not months, of planning. Protection outages (temporarily disabling specific protection schemes, such as secondary relaying, etc.) can put the power system at risk, and planned station outages may require extensive switching in the field. However, functional testing should not be disregarded without proper documentation and clear communication with the customer. 

The advantages of functional testing are numerous. Functional testing not only provides reassurance that the scheme works as intended, but also gives the technician a better understanding of the protection scheme. Verifying interoperability with overlapping schemes is critical and should be investigated thoroughly before placing equipment into service. This reduces the risk of future mis-operations and, if coordinated, can minimize the time spent testing.


Ultimately, as engineers and technicians, we are always trying to balance technical rigor with speed and efficiency to meet the needs of a project. Rather than being rigidly committed to one way of working, flexibility and openness to new methods allow us to continue to improve. While the most critical elements of the protection system or those with a high degree of dependence on outside interactions are better served by functional testing of the larger system, we can identify discrete elements and functions that can be easily verified inside a smaller test envelope. This efficiency can ultimately give us more time to respond to emergent problems on the project or spend longer working on the most critical aspects. Isolating some of these discrete elements also allows us to quickly identify some issues and respond earlier in the process, avoiding potential delays late in the project’s lifecycle.


There are numerous benefits and advantages to implementing functional testing with function testing while performing commissioning activities. We’ve shown that, given the proper coordination and planning, many normal commissioning activities can be combined to make the commissioning process more efficient. This more holistic approach can ensure that each individual element has been verified against its settings. It can also prove that there are no unexpected interactions with other elements internal to that relay and externally elsewhere in the circuit.

By intelligently applying these combined methods, the work of commissioning and testing will be made easier, and we can help design and manage the project more successfully by finding issues as early in the project’s lifecycle as possible. While there is no one-size-fits-all solution to protection system commissioning and testing, examining our practices and finding opportunities to apply the right techniques at the right time gives us our best chance to be successful as well as efficient in our work.

Jacob Loyd is a Relay Applications Engineer with Megger’s TSG Group. His responsibilities include providing technical support and training on Megger’s relay test equipment and Baker motor testing equipment, with particular expertise in the testing and calibration of electromechanical relays. Previously, he spent 16 years at Palo Verde Nuclear Generating Station, with 10 years in electrical maintenance, working on everything from large power transformers to motor control circuits, and six years in the Protective Relaying and Controls group, helping with the transition from older electromechanical relaying to modern microprocessor-based solutions.

Michael Wilson (Mike) is a Senior Relay Applications Engineer at Megger Group. He previously worked as an engineering consultant and NERC compliance manager for a cooperative outside of Chicago and went on to work for TRC Companies and Burns & McDonnell as a Lead Testing and Commissioning Engineer. Wilson earned his BSEE from the University of Missouri–Rolla (now Missouri University of Science & Technology) and his MS in management from Indiana Wesleyan University. He is a student pilot, an IEEE member, and a member of the Experimental Aircraft Association’s (EAA) Chapter 309.