Understanding current standards can protect switchgear users from the risks of internal arc flash events

Electrical workplace safety has always been a critical issue for both manufacturers and users of electrical equipment. It is only more recently, however, that one particular aspect of safety—protection from internal arc flash faults—has been receiving the attention it deserves. Although the probability of an internal arc fault is low, the impact of an arc flash event is devastating. In today’s market, where safety and avoiding downtime are paramount, more users value the importance of internal arc testing.

Example of one of the hazards of an arc flash event

OverviewIn order to assist the user’s decision-making process when assessing switchgear this paper looks at current IEC standards and technical reports; IEC 61439 relates to electrical safety in general and IEC/TR 61641 addresses arc flash testing in particular. It shows how IEC/TR 61641 is limited in scope and is not mandatory. This leads inevitably to manufacturer-specific interpretations. The paper goes on to discuss how it is possible to allow for the limitations and uncertainties of IEC/TR 61641 when specifying switchgear or controlgear assemblies to meet the safety and downtime expectations of manufacturing processes. In particular, we see how the highest levels of protection come from a ‘prevention is better than cure’ approach used together with intelligence that is now being embedded into switchgear. It also takes into consideration the regional variations facing global purchasers.Arc flash safety is absolutely essential for any organization using switchgear to handle its electricity distribution. This is simply because the consequences of an arc flash can be devastating both for personnel and the entire business. Injuries can include serious burns, concussion, collapsed lungs, hearing loss and fractures, and in the worst case, injuries can be fatal. In addition, the damage to equipment and associated loss of production can be extremely costly. Awareness of the risk has risen in recent years, partly driven by a growing dependence on electricity and changes in general legislation related to protection of personnel.We first look at the causes and nature of an arc flash event, before considering the standards available to low voltage (LV) switchgear manufacturers and users. From this, we discuss the practical ways in which users can employ these standards, together with the best available technology, to specify switchgear equipment that meets their particular safety and downtime expectations.

Arc flash hazardInternal free burning arcs in LV switchgear such as a motor control center (MCC) arise when a short circuit occurs and causes a current to flow through air inside the assembly. This current can flow between phases, or between phases and the neutral or ground, or through a combination of all these paths. The amount of energy released depends on the strength of the current and the length of time that it flows. The results can be catastrophic—the internal explosion, consisting of expansion of copper to 67,000 times its original volume, temperatures at 19,000 °C, along with pressure and sound waves can threaten human life.

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Such arcs can result from unfavorable environmental conditions leading to conductive deposits on isolating support elements. Other causes can include vermin ingress or the growth of silver or tin whiskers on exposed conductors. However, a far more likely cause is human error; from tools or excess material left inside the system after inspection, maintenance, or testing. In fact, an estimated 70% of arcs arise from human error. If an arc does occur, the most obvious threat is to any maintenance or operating personnel in close proximity to the arc event site. In addition to the human impact, there are business impacts. In an arc event, all affected switchgear is likely to be permanently damaged. This takes any connected production equipment out of service even if it is fully operational. Beyond loss of production, impacts can be felt in the form of lawsuits, increased insurance costs and lowered common stock values.

Safety is obviously the most critical issue, and for process industry applications, downtime is unacceptable. Therefore, the prevention of arc flash and its consequences is paramount. Every user must understand all the factors that dictate their switchgear’s resilience to arc flash during both operation and maintenance. A natural reaction is to refer to any relevant standards available. However, this approach cannot be relied on for a simple ‘one size fits all’ solution. Although electrical shock hazard protection is definitively covered by IEC 61439, the issue of electric arc protection is addressed by IEC/TR 61641—and, as we shall see, this solution is open to interpretation and is limited in scope.

Product standard IEC 61439

The product standard IEC 61439 is used in testing LV switchgear; Part 1 (General rules) and Part 2 (Power switchgear and controlgear assemblies) are applicable. It is a normative standard that focuses on electrical touch safety and the mechanical ability to withstand short circuit currents, and the dielectric properties and current tracking by means of isolation and insulation of live parts

Technical Report IEC/TR 61641

Although IEC 61439 is a mandatory standard for LV switchgear testing, it does not consider the effects of an arc flash. However, IEC/TR 61641 is an addition to IEC 61439 that includes internal arc testing for enclosed LV switchgear and controlgear assemblies where doors and covers are closed and correctly secured. It assesses safety for personnel and protection for the assembly itself. As a Technical Report (TR) rather than a standard, it is not a mandatory test procedure, but only a guide for testing under arcing conditions. It requires the user’s consent for the way the tests should be performed and how the results should be interpreted.

Users want to be sure that their equipment is safe, reliable, and has high availability. Proving these qualities requires uniform and representative testing. ‘Uniform’ refers to an internationally recognized and standardized test method, while ‘representative’ means that the tests simulate the physical effects that would take place during an internal fault in a real life application. However, the test specimens required by the TR are fully equipped switchgear assemblies, while actual applications comprise a wide variety of configurations. Further variation is created if third-party hardware is added to the switchgear, and also when application-specific protection settings are added. Users must be aware of these issues and confident that their test results sufficiently resemble those that would be produced by the final configuration.

Another issue with IEC/TR 61641 is that it only applies to static operating conditions, with all doors and covers closed and correctly secured. In reality, most incidents occur while operating or as a result of inspection or maintenance when door covers may be open. The first five criteria of an IEC/TR 61641 recommended test assesses the ability of an assembly to limit the risk of personal injury caused by the mechanical and thermal effects of an internal arcing fault.

This is achieved if under testing conditions:

1. Doors, covers, or other access points do not open.

2. Parts do not fly off.

3. Arcing does not create holes in the external faces of the enclosure.

4. Flammable indicators placed around the perimeter of the tested equipment do not ignite

5. Protective circuits for accessible parts of the enclosure are still effective.

The sixth and seventh criteria for an IEC/TR 61641 test address assembly protection and suitability for continued limited operation, which is achieved when:

6. The arc is confined to a defined area where it ignited, without propagation to other areas within the assembly.

7. After clearing the fault, or after isolation or disassembly of the affected functional units in the defined area, emergency operation of the remaining assembly is possible.

The test takes some arc flash–related dangers into consideration; these include the effect of overpressure on doors and covers, thermal effect on the enclosure, effect of ejected hot gases and glowing particles, and electrical touch safety. However, it does not cover damage to internal partitions or other effects that may constitute a risk, such as toxic gases or noise.

As well as providing this guidance for assessing arc flash effects, the TR also specifies the characteristics of the arc itself, in terms of four different arc scenarios: 1) either the arc exists for the full duration of the applied power, 2) the energy to the arc is interrupted before the applied power is removed, 3) the arc dies through self-extinction, or 4) initiating an arc is rendered impossible by using an arc-free design.

The first scenario involves the maximum amount of energy and potential for damage to the equipment. In the 300 ms typical of a test, the complete switchgear assembly can be ruined, while its supported processes come to a standstill. It must be noted that complete destruction of the switchgear can be prevented by adding to the design the capability to interrupt the short circuit current as soon as possible after ignition, in an effort to limit the destructive effects to the compartment of origin.

Various methods of achieving this interruption exist. The primary method involves interrupting the short circuit current by using an overcurrent protection device. The faster the device can act, the lower the “let through” energy available to feed the damaging arc. Circuit breakers and fuses offer differing levels of performance; a molded case circuit breaker (MCCB) may take 3–15 ms to interrupt the current, while an air circuit breaker (ACB) in instantaneous mode requires 40 to 50 ms. While fuses act faster than MCCBs or ACBs, especially with higher kA fault currents, there is no dedicated ms range for current interruption.

Another interruption method involves arc-quenching devices that react to the light of an arc flash and overcurrent in the main circuit and switch the fault current to a controlled circuit. Their total reaction time depends on the type of circuit breaker and the protective relay used in the incoming circuit. It should be noted that the efficacy of this method is entirely dependent on the health of each of its numerous components.

Another approach is to place barriers and shields in a way that arcs will self-extinguish within a few milliseconds. This can be accomplished by placing insulating barriers and shields between phases and between phases and ground. Arc tests will have to verify this type of design.

The fourth scenario is based on a fundamentally different concept centered on prevention rather than cure. Whereas the previous methods are concerned with minimizing the effect of an arc if it occurs, this concept—arc-free design—exists instead to prevent an arc from occurring in the first place.

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A more recent approach is to build in arc prevention through arc-free and arc-proof zones. In an arc-free zone, all live parts are fully insulated, while in an arc-proof zone they are fully enclosed by segregating barriers. When testing, it is not possible to apply a fuse wire between any poles and earth to initiate an arc in an arc-free zone without damaging the insulation system. Under the TR’s guidelines, arc-free zones do not require testing, but arc-proof zones do.

A well-designed system ensures that the entire area, including the bus connection and the power connection, all the way to the breaker, is designed such that an arc will never happen. This paper represents a very strong argument in favor of users demanding insulated busbar in switchgear systems.

Interpretation of IEC/TR 61641

We have seen how the IEC standard provides a framework for designing and testing for electrical safety in LV switchgear. However, we have also seen how the TR has limitations, with room for interpretation. So what are manufacturers actually offering in today’s market?

Currently, there are now two schools of thought. One embraces the concept that arcs must be prevented from happening in the first place, whether the system is in normal operation or under ‘doors open’ maintenance or inspection conditions. The other does not invest heavily in absolute arc prevention, but assures safety through mitigation techniques that minimize the effects should an arc occur.

With pressures to ensure safety and prevent downtime, process industries will invest to reduce risk as much as available technology will allow, and they likely share the view that:

Risk = Probability x Effect (or damage)

To minimize risk, companies will take every possible measure to eliminate or minimize both its probability and effect. For this reason, many favor the ‘preventative’ approach as applied by Eaton’s new Power Xpert CXH™ motor control and power distribution center. The power path through the Power Xpert CXH MCC can be isolated by routing it through arc-free and arc-proof zones as described above. If the entire path is isolated in this way, the MCC becomes an arc-proof assembly. This approach also has the advantage that it does not introduce a risk of ‘over-protection’ causing nuisance tripping of protective devices.

Manufacturers offering systems that are arc-proof, not arc-free, need to provide evidence through testing that the areas indicated as arc-proof are, in fact, arc-proof. It is best for the user to specify that evidence is needed that all arc-proof areas have been tested.

Safety during maintenance

Alongside the possibility of an arc during normal operation, arcs can occur during or after maintenance. It is, however, possible for the most modern systems to be designed with protection against ‘maintenance’ arcs that are often attributable to human error.

One problem for electrical maintenance personnel is that switchgear maintenance is not covered in IEC/TR 61641, which only deals with “normal operation” where the cabinet doors are closed. Another problem is that when conducting maintenance, in an effort to maintain production uptime, operators frequently don’t want to shut down an entire panel. This can present a potentially dangerous situation where personnel may work on partially energized equipment.

In modern systems, if an arc occurs, short circuit protection devices shut the power off. These devices are cascaded from the main incoming breaker on the main distribution board to incomers on sub-distribution boards to MCCs and to individual outgoing circuits. In a well-coordinated system, the circuit breaker closest to a short circuit and its resulting arc should trip to clear the fault. If an arc occurs at the load side of a MCCB, the MCCB will trip within a few milliseconds and limit the available energy. Systems like the Power Xpert CXH are designed to withstand the let-through energy and meet IEC/TR 61641 in normal operating conditions, with closed doors.

If the arc is not on the load side of the MCCB, but on the line side, the incoming air circuit breaker might be expected to trip much more slowly (up to 500 ms), letting through much more destructive energy. The damage that can be caused by the thermal energy of an arc flash of 300 ms duration is comparable to the explosion of a hand grenade.A

Modern switchgear is designed to provide additional means to deal with these situations. As an example, during maintenance, a properly equipped air circuit breaker can be set to trip faster than the standard instantaneous setting and limit the let-through energy to its lowest possible level. This is possible using trip units equipped with an Arcflash Reduction Maintenance SystemE.

Arcflash Reduction Maintenance System reduces available incident energy

A http://www.esfi.org/index.cfm/page/Arc-Flash-Awareness/pid/10862

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Arc-free and arc-proof zones

Eaton’s philosophy is that the best way to mitigate the risks of internal arcing is to prevent the arc from happening in the first place.1 Arc-proof connection points and copper bars2 Arc-free copper bars and connectors3 Arc-free main busbar system4 Arc-free distribution busbar system5 Arc-free connection from vertical bus to protective device6 Arc-proof functional unit

otee:N Zones 1 and 6 have been DEKRA witness tested for forced arc ignition and passed in categories 1 to 7.

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As operators will testify, while a system is being maintained, one of the most critical moments is when an outgoing unit is being re-inserted and connected to power. In this example, maximum safety can be achieved if the outgoing compartments are shielded from one another so that an arc in one compartment does not travel to another.

IEC/TR 61641 criterion 6 allows switchgear manufacturers to specify the “defined area” to which an arc must be contained. The very safest systems are designed so that a single functional compartment (i.e., a withdrawable motor starter) is the “defined area” according to IEC/TR 61641. Importantly, specifiers and operators need to be aware that some systems may have an entire switchgear structure or section defined as its functional unit, which may not meet the expectations of the user.

One other factor to consider is that the sealing of functional units has a negative effect on thermal behavior. The more you seal, the hotter the equipment becomes. To achieve the best combination of thermal performance and safety, heat build-up and dissipation should be considered from the design of the components to the design of the system. Users should consider specifying defined areas for confining arc faults while understanding what the implications are on the thermal performance of the system.

The final point to consider related to safety and maintenance is that the most critical moment after maintenance is when the functional unit is reinstalled and powered up again. In conventional systems, the operator has to open the functional unit compartment door in order to insert the drawer. With the compartment door open, electrical contact is made while the user is reinserting the drawer and standing in front of the switchgear. If an arc occurs at that moment, on the line side of the outgoing breaker, the arc explosion can be catastrophic as the user is exposed to the full force of the impact.

The best design for safety is to be able to insert the drawer with no electrical contact, close and lock the door, and then rack the drawer into its service position. The ability to rack behind a closed door ensures the safest possible operation as it is only possible to make electrical contact when the door is closed and locked. There are two ways to reach the device, 1) with a hand-held specialized crank used while standing at the system, or 2) a remote racking device that can be actuated from a safe distance outside the threat area.

Intelligent switchgear components

Recent developments in embedded computing and communications technologies are also allowing manufacturers to develop further tools for driving down risk. Groundbreaking motor control center technology is now embedding products such as Eaton’s Power XpertT C445 electronic motor management relays. The ultimate objective is to reduce the need for maintenance personnel to work in electrical rooms on systems under live conditions. A much better alternative is to display essential diagnostics and maintenance information on user interface displays installed in safe areas. This can be easily done via industry standard communication protocols and infrastructure.

Eaton’s C445 motor management relay measures electrical parameters and provides them to the user either via an interface on the door of the drawer, or via a network connection for remote access. In-depth data analysis can be done via PC software connected via USB.

With the C445 motor management relay, Eaton’s Power Xpert CXH MCC can be integrated into plant-wide monitoring and control schemes. The range of integration possibilities includes all varieties of PLCs and DCS systems through networking protocols including ModbusT RTU, PROFIBUST, Modbus TCP, and EtherNet/IP.

Conclusions

We have seen how IEC/TR 61641 provides the guidelines for testing for arc flash protection, yet it does not provide a set of mandatory or definitive tests. It also does not cover arc flashes that can occur during “open door” maintenance operations. Instead, for the safety of their personnel and equipment, users must ensure that their equipment has been tested to the applicable standards.

Understanding current standards makes it possible to better specify equipment that meets or exceeds the requirements of IEC/TR 61641. Of course, real-world applications are subject to both environmental and human interaction factors; therefore, it is difficult to claim that the risk of an arc flash has been completely eliminated. Nevertheless, this ideal state of “zero risk” can be approached by specifying the correct equipment. Users deserve equipment that has been subjected to suitable test procedures and carries the testing agency documentation to prove it.

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