Arc flash accidents in electrical installations have become a growing concern. While most types of electrical injuries have been significantly reduced as a result of improved technology and safety, arc flash incidents have emerged as one of the leading causes of serious injury or even death among electricity workers.
Are you prepared against arc flashes?
It doesn't take much to set off an arc flash, but it can result in a devastating explosion that can maim or kill, destroy equipment, and cause a facility to be shut down for weeks. The primary line of defense against arc flashes is usually prevention, but when they do occur, most electrical systems are unprepared to deal with them because the traditional techniques for dealing with flashes are primitive.
Conventional arc-resistant switchgear is designed to muffle the flash rather than eliminate it by exhausting its energy through a chimney or plenum, providing no protection for equipment. Worse, its effectiveness can be severely harmed if a cabinet door is open or a panel is turned off, as is common during routine maintenance. This article is not about internal arc-proof assemblies, but about how active protection systems can be used to mitigate the effects of an arc fault condition when it occurs.
What's an arc flash?
When the insulation between electrical conductors is no longer sufficient to contain the voltage within them, an arc flash occurs, which is a strong electric current and often a large explosion that passes through air. This causes a short circuit, allowing electricity to flow from conductor to conductor. Arc flashes can produce heat up to 20,000° C, generate audible noises exceeding 160dB, and emit ionizing radiation in addition to molten material from the source.
In a paragraph titled “Two Deaths Per Day” Fire Engineering magazine states: “An arc flash is an electrical release of energy hotter than the surface of the sun and capable of exploding with the strength of eight sticks of dynamite. It kills two workers a day, every day, year in and year out; arc flash injuries occur 1,000 times more often than a shark attack. A shark attack receives front-page coverage in the newspaper; an arc flash fatality doesn’t make the news at all.”
What causes a flash?
Human error is the leading cause of arc flashes, with 65% occurring while an operator is working on the switchgear. Here are some examples:
- Overtightened unions, loose/dry joints, series arcing, and thermal runway
- Animal/insect intrusion - these little buggers are always looking for a nice small warm home.
- Foreign objects left behind during construction or maintenance - tools can be left behind.
- Inadequate or incorrect maintenance - condensation under MCC top tiers is a classic cause of rust, which will eventually scale off and fail.
- The life cycle of aged switchgear and cabling is generally exceeded, which is exacerbated by poor and misinformed maintenance, and breaking capacity degrades over time.
- Opened compartment doors on live switchboards, introducing moist/dusty/salty air, which can result in a lower impedance between two points.
Why get arc flash protection and what are the necessary standards?
The Work Health and Safety Act of 2011 states that it is the responsibility of anyone conducting a business or undertaking at a workplace to keep their employees safe and prevent a potentially catastrophic event from occurring. The employer may be liable under the act if an arc flash accident occurred and the employer had reasonably practicable means of preventing it.
Although there is no specific standard in Australia for arc flash calculations, IEEE 1584 is used to determine the incident energy and arc flash boundary for electrical equipment installed. Arc Flash studies for coal mines have become a requirement of AS3007.
Reducing the Risk of Arc Faults
Although active protection will not prevent accidents, it will significantly reduce the damage caused. There are electronic devices available that work by detecting overpressure or the bright light produced by the arc to monitor and limit it when it occurs.
The first method employs arc detectors that detect cabinet overpressure. As a result of the arc's rapid energy release, a pressure wave of such magnitude develops, which, if not contained, can cause switchboard structural failure or even propel missiles such as panel doors. As a result, pressure sensors capable of signaling the pressure peak associated with arc ignition with a delay of about 10 to 15ms can be installed.
Because it operates directly on the supply circuit-shunt breaker's opening release, such a system does not require any electronic processing device. It is critical that the device be set at fixed trip thresholds so that the arc detector intervenes when an internal overpressure is reached.
ABB Arc Fault Detection Technology
It all comes down to the time when it comes to mitigating the effects of arc faults. The graph above depicts the detection ability of the ABB Arc Guard system (TVOC-2) in relation to arc fault energy and stages. It is important to note that the time and thus energy at the time of detection and trip are well before enough energy can pass to cause a cable fire.
The issue with this method is that it is difficult to predict the amount of overpressure generated by an arc fault inside a switchboard, which leaves switchboard safety unwarranted.
The second method entails installing detectors that detect the light flux produced by the electric arc. With a reaction time of about 1ms, the arcing control system detects the event and sends a tripping signal to the circuit breaker.
Arc fault events can be significantly mitigated by strategically installing detection and anomalous current sensors in conjunction with a properly sized main circuit breaker.
These combined sensing systems accurately and consistently cut power to the fault within 35ms. To provide even more protection, the detection system can be distributed through the sub-breakers at each sub-board and tiered back across each of those breakers to provide a coordinated shunting trip in order to avoid a cascading arc condition.
Solution Installation Tips
The ABB Arc Monitor is designed to fit all types and sizes of low- and medium-voltage switchgear thanks to its modular design. Here are some pointers on what components are needed and how to install them.
ABB Arc Guard System
ABB TVOC-2 Arc Guard System™ is SIL-2 certified according to IEC 61508 and IEC 62062 providing an unrivalled optical arc flash mitigation device with the fastest reaction time on the market. The TVOC-2 identifies the light from an occurring arc flash and transmits the signal to the breaker within 1ms. Along with ABB’s Emax 2 circuit breaker, the total arc fault clearing time is less than 50ms. The current sensing unit CSU-2 is an accessory to the TVOC-2 which will identify the current increase associated with an arc flash.
The ABB TVOC-2 Arc Guard System™ has a protection degree of IP54 with an HMI touchscreen that can be panel door mounted. It can be expanded with up to 30 optical sensors for a wide coverage from a single TVOC-2 and can be monitored remotely via Modbus RTU communication protocol. Its user-friendly start-up menu and calibration-free installation on either a DIN-rail or a wall mount makes it a quick solution with ensured reliable function.
The Arc Guard System™ monitor can be mounted anywhere in the switchgear, such as in the breaker cubicle or in a separate control cabinet. A separate tripping circuit handles tripping. This method allows you to connect up to three breakers and, if necessary, trip different breakers depending on where the arc occurs. If the supply voltage fails, energy is stored in the unit for up to 0.2 seconds of operation. Even if the voltage disappears at a short-circuit fault, this is enough to close the tripping circuit.
CSU (Current Sensing Unit)
The CSU is an accessory used if you cannot prevent direct sunlight or other highly intensive light reaching the sensors frequently. CSUs can be mounted in series if more than two are needed.
The CSU measures either 1, 2 or 3-phase. Three-phase is, however, preferable for reasons of safety and reliability. Current transformers with a secondary current of 1, 2, or 5A are used for this purpose.
Detectors
Detector cables range in length from 1m to 60m, and each detector is made up of an optical cable and a lens that are calibrated together to provide the same sensitivity regardless of cable length. Each Arc Monitor can support up to 30 detectors, and because they are Fibre-optic sensors, they are not affected by electrical noise.
The basic strategy for sensor positioning is to ensure that all parts that may be affected by an arc are covered. This usually entails the horizontal and vertical bus bar system, as well as the breaker cubicle. If at all possible, it is also preferable to supervise each cubicle.
Why should you invest in an Arc Guard System?
- Save lives
- Save your equipment
- Reduce downtime
- Extend the life of switchgear
- Spend less money on repair costs
Ensured for the future
ABB's TVOC-2 Arc detection device, which has SIL-2 safety certification to IEC 61508 and IEC 6206, is saving lives. Its solid construction and reputation give those in the electrical industry peace of mind.
Browse our range of ABB Arc Guard Systems™ & Arc monitors here. If you would like to find out more information about our products, get in touch with us on 1300 556 601 today.