Electrical arcing
By TL Martin
All of these can be defined as an uncontrolled intense luminous discharge of electrical energy that occurs when electric current flows across an insulating medium.
In other words "arcing" occurs when electric current flows uncontrolled accompanied by ionization of the surrounding air from phase to ground, phase to neutral, and/or phase to phase. The intense light and heat energy at the point of the arc is called an "arc flash".
An ARC builds up in milliseconds and releases a vast amount of energy.
Insulation failure (caused by defective or aging insulation material), poor or incorrect maintenance, ingress of dust, moisture or vermin and human errors (touching a test probe to the wrong surface or a tool slipping and touching live conductors) are the most common causes of arc faults.
Arc faults are particularly dangerous and potentially fatal to personnel. During an arc fault enormous energy is released that causes serious injuries to personnel in close vicinity such as severe burns, ruptured eardrums, collapsed lungs, damaged eyesight and in some cases death. Accident reports have revealed that Electricians have been injured even at 3 or more meters away from the arc point. Arc faults produce large shock waves that can blow personnel off their feet and there is also the risk of being exposed to flying debris
Temperatures at the arc point can reach as high as 20 000 °C or about four times the temperature of the sun's surface. The intense heat vaporizes the copper and the copper vapour expands to 67 000 times the volume of solid copper. Metal is blasted and splattered from the arc point. The volume of hot ionized gas produced, increases proportionally with energy. It is thus crucial to remove the source of energy as soon as possible in order to eliminate the avalanche effect of an arc flash fault.
To see a video of an arc flash incident captured by a security camera, follow this link: Arc flash incident
Here is a NFPA review of the incident:
NFPA reviewThe answer is simple. Yes! Most 400V electrical systems have sufficient electrical energy capacity to cause an arc flash hazard. Medium-voltage equipment (above 1000V) has higher energy and therefore a bigger potential for arc flash hazards.
As an example of the energy released in an arc flash incident, consider a single phase-to-phase fault on a 400V system with 25kA of fault current. The resulting power is 10MW. If the fault lasts for 10 cycles (200 milliseconds) at 50 Hz, the resulting energy would be 2 mega joules. For comparison, TNT releases 2175J/g when detonated. This fault energy is thus equivalent to 919 grams of TNT. The characteristic of an arc flash blast is quite different from a chemical explosion (more heat and light, less mechanical shock), but the resulting devastation is comparable. Using the figures above, let's increase the arc duration time from 200 to 500 milliseconds: the power is still 10MW but the resulting energy is now 5 mega joules or 2,3kg of TNT. From these elementary calculations one can clearly see what effect the duration of the arc fault has on the energy released, the duration of the arc fault is directly proportional to the amount of energy released. The rapidly expanding superheated vapor produced by the arc can cause serious injury or damage and the arc emit an intense ultraviolet and infrared light.
Figure 1, below is a graphical representation of the relation between arc duration and the amount of energy released based on 10MW of electrical power (as per example above).
The duration of the arc fault is mainly determined by the time it takes for overcurrent or earth fault protective devices to detect a fault, send a trip signal to the circuit breaker and for the circuit breaker to subsequently disconnect the source of energy.
Fast acting fuses may disconnect the circuit from the source of energy in 8 ms or faster, when subjected to high short circuit currents usually appearing in 3 phase symmetrical bolted cases, while other devices may take much longer to operate and remove the source of energy. The problem with unbalanced, single-phase and high impedance fault currents is that they are lower than 3 phase fault currents. Therefore, protection devices may not necessary detect and limit let thru current and will require more time to clear the fault.
In some cases the protection devices at main distribution points are graded with downstream protection systems. This means that there may be considerable time delays before the normal protection at the main distribution point operates to allow downstream protection devices to clear the faults first.
The arc detection and protection equipment manufactured by SELCO A/S is specifically designed to detect an arc flash and to issue a trip (almost instantaneously) to the main circuit breaker. The duration of the arc fault is thus reduced to the mechanical opening time of the circuit breaker. The operating time of modern circuit breakers is in the region of 50 to 80 milliseconds.
This arc protection equipment may be installed as a separate protection scheme that operates independently of, or in conjunction with the normal overcurrent and earth fault protection schemes. It would be possible to incorporate the arc detection equipment with your existing overcurrent and earth fault protection relays.
Figure 2 below shows an arc fault that lasts for about 500 milliseconds while figure 3 shows an arc fault that lasted for about 100 milliseconds.
Although human errors are one of the most common causes of arc faults when performing work on live equipment it is possible to protect personnel from some of the arc flash hazards by ensuring that they are wearing proper personal protective equipment (PPE). PPE includes clothing, gloves, insulated tools, face protection, and glasses.
Note: until equipment is placed in a safe work condition (disconnected and properly isolated from all sources of electric energy), it is considered live. This means that conductors and equipment are considered live when checking for voltage while putting equipment in a safe work condition.
So, we think that when electricians (trained personnel) perform maintenance duties they are to a certain extent protected against the hazards of an arc flash by wearing appropriate PPE (PPE should be viewed as the last line of protection). There are of course other potential hazards evolving from an arc flash where PPE will not provide adequate protection to personnel: During an arc flash the explosive expansion of the surrounding air can create pressure waves able to blow personnel off their feet. Total force on a person standing in front of an open enclosure can well exceed 4 400N (450kg). Such forces crush a person's chest, breaking bones, puncturing lungs and other organs. Forces like this can even propel workers into walls, windows and other equipment. Falling from ladders or other unstable and non-secure positions can also injure workers. Even if the blast is not powerful enough to propel the worker, arc fault tests indicated that severe head tossing often occurs. These cause whiplash type injuries with the possibility of brain and/or spinal column damage.
What about the other common cause of arc faults, device and insulation failures? This can happen at any time. It may even occur when other personnel such as operators, contractors etc. walk through or perform other duties inside the switchgear room. These personnel will most probable not wear the correct PPE that will protect them against the potential hazards of arc faults.
The best solution is thus to install a dedicated arc detection and protection system.
According to the Occupational Health and Safety Act and Regulations 85 of 1993, the Minister of Manpower has under section 35 of the Machinery and Occupational Safety Act, 1983 (Act No. 6 of 1983), made the regulations specifically pertaining to Schedule 6 as follows:
"2. Safety equipment. - Without derogating from any specific duty imposed on employers or users of machinery by the Act, the employer or user shall provide free of charge and maintain in good condition insulated stands, trestles, mats or such other protective equipment as may be necessary to prevent accidents, for use by persons engaged in working on or in close proximity to live electrical machinery or dead electrical machinery which may become live."
"6. Electrical control gear. - (1) The user shall provide every electrical installation and power line with controlling apparatus and protective devices which shall, as far as reasonable practicable, be capable of automatically isolating the power supply in the event of a fault developing on such installation or power line."
The following has been extracted from the South African National Standard, SANS 10142-1:2006 (previously known as SABS 0142). This standard specifically covers the wiring of low voltage premises and part 1 deals with low voltage installations. This specification:
"Applies to electrical installations of residential premises, commercial premises, public premises, industrial premises, prefabricated buildings, fixed surface installations on mining properties, construction and demolition site installations, agricultural and horticultural premises, caravan sites and similar sites, marinas, pleasure craft and house boats, medical locations, exhibitions, fairs and other temporary installations, extra low voltage lighting installations, electrical installations for street lighting and street furniture, and equipment enclosures (structures that provide physical and environmental protection for telecommunication equipment). Covers circuits supplied at nominal voltages up to and including 1 000 V a.c. or 1 500 V d.c."
"The aim of this part of SANS 10142 is to ensure that people, animals and property are protected from hazards that can arise from the operation of an electrical installation under both normal and fault conditions. An electrical installation has to provide protection against: shock current, overcurrent, fault current, overvoltage, undervoltage, excessive temperatures, and electric arcs. If any of the above arises, the protection should automatically disconnect the supply or limit currents and voltages to safe values. In the case of undervoltage, the protection should ensure that dangerous situations, due to the loss and restoration of supply (for example, to a motor), or due to a drop in voltage, cannot occur."
Various regulations have been put in place to primarily protect the live of humans from the effects of arc faults.
To summarize: It is the responsibility of both employers and their employees to do their best to create a workplace for electrical workers that is not just safer but puts in place the best possible processes and procedures that are fully understood, practiced and enforced for optimal results.
The arc flash detection and protection equipment manufactured by SELCO A/S comprehensively protect the functional reliability of the distribution board and at the same time satisfies all compulsory regulations regarding the protection of personnel.
As mentioned before, the consequences of an arc flash in a power system can be disastrous. Within a very short time (milliseconds) after a fault occurred, pressures and temperatures rise to levels that can easily destroy the metal housing surrounding the equipment. The energy released by such a fault is enough to cause sheet metal to melt or evaporate and serious fires could follow.
By installing reliable and fast acting arc flash detecting and protection equipment in the electrical switchgear and distribution systems, plant owners will enhance the safe operation of the equipment. Arc detection equipment protects both personnel and equipment and reduces repair and reconstruction costs. System downtime is shortened due to the fact that the amount of potential damage caused by the arc flash is limited. Therefore the total cost of ownership is drastically reduced. By installing and maintaining arc protection equipment the possibility exist that insurance premiums could be reduced and a potential reduction in workers compensation and insurance claims.
One specific application for the SELCO A/S arc detection and protection system is in the marine industry.
Due to the fact that marine vessels cruise in isolation on open waters the personnel onboard perform some maintenance and repairs to the switchgear in order to maintain operation of the vessel. Arc faults and the devastating consequences thereof is something that everybody wants to avoid especially on the open sea.
SELCO is a pioneer in the field of arc detection and protection and has gained experience since 1962. The arc-detecting relay is connected to light detecting sensors. These arc-detecting sensors consist of two silicon solar cells encapsulated in transparent polyester. The sensors are strategically placed in various cubicles or drawers inside the switchboard.
It is possible to add an overcurrent relay that could monitor the electric current flowing to the switchboard. Although this is not a requirement for the arc system to operate, this option will increase the reliability of the system (minimize spurious tripping). The theory behind this is that when a genuine arc fault occurs the electrical current feeding into the fault will increase drastically. Two conditions need to be fulfilled before the trip is send to the circuit breaker: a certain current flow that exceeds the normal operating current of the system (the threshold level is adjustable from 1.5 to 3.0 x In) and a signal from the arc detector, implying that the sensor has reacted to a high intensity light source.
This system also offers a circuit breaker fail function which makes it possible to trip the upstream supply breaker after a time delay of 50 - 150 milliseconds if the overcurrent and arc signal remains after the incomer circuit breaker was suppose to open and clear the fault (meaning that the circuit breaker has failed to trip.)
There are two types of arc detectors available: 180° or 360° characteristic. A 5m or 10m screened cable (length of cable to be specified at order placement) and mounting bracket is supplied with each sensor. An optical cable sensor will also be available from SELCO in the fourth quarter of 2009.
One of the main advantages of the SELCO arc detection system is that the condition of the sensing circuits is continuously monitored.
The arc detection and protection system supplied by SELCO A/S is simple to install and straightforward to commission. This makes the system ideal for installation on existing switchboards.
The system is designed to operate on a 48V - 220V DC supply (fed from station batteries). An arc-detecting relay is also available for a 220V - 250V AC supply.
* The D1000 Arc detection relays shown in this diagram will only be available in the fourth quarter of 2009.