Passive and active ARC fault protection in LV switchgear

February 10th, 2015, Published in Articles: Vector

The traditional methods of passive arc fault protection and the latest active protection options and applications available to industry.

Having to wear a “moon suit” against arc flashing when entering LV switch rooms is new to South Africa and many companies who have never experienced arc flash incidents are awaiting developments in the occupational health and safety (OHS) regulations.

Many workers who must wear category 3 or 4 arc-rated suits find these very uncomfortable. It is felt that these suits create their own hazards and many companies are looking for options to reduce the level of personal protective equipment (PPE) when operating and working with LV switchgear.

Traditional method: passive protection

With the development of metal clad switchboards came the dangers of internal arcing faults. Arc fault behaviour and its impacts on enclosed switchgear were studied and internal arc fault tests were conducted in the 1950s.

In the early 1960s, some LV motor control centres (MCCs) were designed and tested for internal arc fault containment (IAC) in some countries.

LV switchgear: arc fault test guidelines

The test guidelines for IAC were established in the 1950s. The essence of the pass criteria is that a person standing in front of the switchboard should not be harmed by any by-products of an internal arcing fault. These could be hot ionised gases or any projectiles coming from the switchboard and the resulting overpressure.

In the 1950s, arc fault tests were conducted to the German PEHLA recommendations and, in 1996, the international LV IAC guideline IEC/TR 61641 was published. The current South African LV switchgear standard SANS 61641 (2008) includes IAC testing guidelines. In brief, these guidelines are as follows:

  • The switchboard to be tested is connected to the test station’s high fault level busbars.
  • Cotton indicators are placed in front of the switchgear.
  • An ignition wire is connected across all three phases in the module under test.
  • The test station is calibrated to deliver the specified prospective fault current and the station circuit breaker is usually set to trip after 300 ms.

When conducting a standard test, the arc is initiated after the short circuit protection device (SCPD) in an outgoing functional unit. In the more rigorous “special tests”, the arc is initiated on the line side of the outgoing SCPD or on other line-side areas such as the incomer or main busbars. The purchaser must specify the points of ignition (see Fig. 1):

  • Point (a): load-side of the SCPD.
  • Point (b): line-side of the SCPD.
  • Point (c): distribution bus.
  • Point (d): incomer.
  • Point (e): main bus.

Fig. 1: The points of ignition must be specified.

Danger of limiting requirements

Many experts recognise the danger of limiting requirements to the load side by stating that “special tests may be required to simulate arcing faults initiated in the compartments tested by failure of a protective device itself. Failure at the incoming terminals or connections to a protective device from the main supply or where the connections are not adequately shrouded. The transfer of an arcing fault from the load side to the line side of a protective device”.

South Africa has a long tradition with IAC LV switchgear and this could well be the reason for the comparatively limited but growing number of incidents of arc burn injuries in LV switchgear in this country, compared to developed countries such as the USA. However, one should also consider that not all incidents are always reported in the South African market. Nevertheless, the dangers of arc flash incidents should not be dismissed out of hand. Arc flash risk assessments are a crucial safety requirement and should be carried out even if the switchgear is IAC.

To allow workers to operate LV switchgear without having to wear “moon suits”, those responsible for occupational health and safety need the necessary IAC test reports to support this. Test documentation should not only include the outgoing functional units but all compartments to verify that the complete switchboard has been tested for IAC.

Regulations, standards and OHS requirements

IEC 61641provides the internal arc fault containment testing guidelines but IAC is not yet mandatory in South Africa.

IEC 61641 suggests that IAC is an enhancement of internal separation and is not to prevent the initiation of an arc when undergoing maintenance. It is very important to realise that IAC does not provide switching operators or maintenance staff any protection if any doors are open or if any covers are not closed properly and fixed in place.

The guideline regarding “safe working on low-voltage electrical installations” warns that persons working on or near exposed energised conductors should be aware that arcs produced under fault conditions can cause severe flash burns and injuries as a result of flying debris. In particular, it points out that overcurrent protection may not safeguard the worker.

The US IEEE1584 2002 Clause 4.1mentions strategies to minimise burn injuries:

  • The specific rating of PPE.
  • Working de-energised.
  • Installing arc-resistant switchgear.

The US NFPA 70E-12 Hazard Risk Category 13(C) (15) (a) does not require the use of arc-rated PPE on IAC switchgear when carrying out normal operations.

The OHS codes and practices in both the US and South Africa are trending towards no live work. The definition of “live work” is still under debate. For example, many feel live work includes testing, fault finding and conducting thermography. If these activities were to be outlawed as they require opening a door on a live switchboard, even if you are wearing arc -rated PPE, this will have a major impact on how plant are managed. No live work will impact on the reliability, availability, maintainability and safety of the plant.

In summary: passive arc fault protection

While internal arc fault containment seems to be universally accepted as the leading strategy to protect workers against arc burn injuries, the testing is performed with all doors and covers securely closed.

If you consider fully withdrawable swtichgear where all operations are performed with the compartment doors closed and the complete switchboard has been tested for IAC, then the arc flash risks are minimal.

Fixed or demountable construction LV switchgear

Working inside a starter compartment on an energised MCC is extremely hazardous because of the possibility of live connections. With the door open there is no IAC.

Many people feel they have no choice but to work live and opt to wear arc-rated PPE to provide some protection against possible arc explosion.

Additional arc fault testing with open doors

To simulate a possible working environment, ABB conducted arcing fault tests generally in accordance with EC 61641 with the starter door open, using a mannequin dressed in Cat 2 PPE instead of the regulation cotton indicators.

Test: Fault on the load side of the starter MCCB (“standard test”)

Possible malfunction simulations include:

  • The door interlock may fail.
  • The operating handle on the motor isolator may slip and the contacts remain closed.
  • The electrician may defeat the isolator interlock and accidently make contact with live terminals.

The result was that the starter short circuit protection, an ABB 400 A MCCB, tripped in 3,55 ms. The let-through energy on the white phase was 863 kA sq. secs. The Cat 2 PPE was not damaged.

Test: Fault on line-side of the starter MCCB (“special test”)

Possible malfunction simulations include:

  • The protection fails. The starter MCCB contacts are welded, the starter remains live and the MCCB will fail to trip.
  • The protective line side shrouding on the MCCB has been removed and the electrician accidentally makes contact with the live terminals.

The result was that the incoming ABB E3H-25 air circuit breaker with PR121/P protection release tripped in 89,8 ms. The let-through energy was 234 mA sq. secs. The Cat 2 PPE suffered severe burns.

Active arc fault protection options

Active arc fault protection devices may be used in LV switchgear which is not designed and constructed for internal arc fault containment (IAC). When these devices are fitted, the switchgear should be tested to the IAC guidelines mentioned here to verify arc fault protection. Active protection devices could also be used to provide protection when the doors are open and we now examine this possibility and other active arc fault protection options:

Short-circuit protection on the LV switchboard incoming supply

The additional arc fault testing conducted by ABB demonstrated that incomer short-circuit protection does not operate fast enough to provide personnel with protection.

The following should be considered with this option:

  • Discrimination with downstream breakers must be considered.
  • Motor in-rush currents must be considered.
  • Reduced equipment damage. Short-circuit protection will limit the harmful effects of an internal arc. However, unless the switchgear has been tested for IAC, arc-rated PPE maybe still be required.

Arc detection systems

This technology has been used for over 35 years to provide active arc fault protection in LV switchboards. A common application is to locate the detectors in compartments which have not been arc fault tested, such as cable compartments and special, custom-made compartments.

Fig. 2: Arc detectors with UFES.

Fig. 2: Arc detectors with UFES.

One of the problems faced here is the difficulty in discriminating between the light emitted from a “normal” short-circuit switching operation and a real fault arc incident. If the arc detector is located in a motor starter where a circuit breaker is used for short-circuit protection, there could be a sudden increase in light from the breaker (arc chamber) when it interrupts a short circuit.

This can result in nuisance tripping of the arc detection protection.

The intense light associated with an arc is detected by the arc detector. A current sensing unit can be provided to verify that there is also an arcing current. The arc monitor will react in around 2 ms, sending a trip signal to the circuit-breaker to clear the arc. The breaker clearing time is critical to provide personnel protection which can be as high as 100 ms.

Arc detectors in conjunction with ultra-fast earthing switch (UFES)

The NFPA 70E-12 Hazard Risk Category 130(C) (15) (a) specifies that, for non-IAC LV switchboards up to 65 kA, where the arc duration is no longer than 30 ms, arc-rated PPE is not required to perform normal operations.

Traditional arc detection systems as described here rely on a circuit breaker to trip and eliminate the arc voltage. With commercially available air circuit breakers, the typical clearing time is greater than 30 ms. Nowadays, it is possible with the latest technology to eliminate the arc withm 5 ms using an ultra-fast earthing switch (UFES) system. If the arc can be eliminated within 5 ms, there is very little arc pressure, let alone any hot ionised gases.

The arc detection system initiates the operation of the UFES which, in turn, creates a phase-phase/phase-earth bolted short-circuit (and therefore 0 V), thereby extinguishing the arc. The incoming breaker trips on instantaneous overcurrent, isolating the system:

  • The intense light associated with an arc is detected by an arc detector (QRU) which operates the high-speed earthing switch (UFES) (see Fig. 2).
  • The earthing device short­circuits the bus, eliminating the arc voltage and extinguishing the arc under 5 ms.
  • The incoming breaker (CB) trips on instantaneous overcurrent, finally clearing the fault.

UFES active arc fault protection technology can be used in a variety of applications:

  • LV and HV applications up to 40 kV; 65 kA/s new and old installations; switchgear; capacitor banks, drives etc.
  • Non-withdrawable LV switchgear where it is not possible to remove the functional unit for maintenance: a “maintenance mode” could be used to activate the arc detection system when working with the door open – this eliminates the nuisance tripping scenario. The benefit of this solution is that it can be retrofitted to old switchboards where it may not be economical to replace with a new IAC switchboard. This could also be a useful addition for IAC non-withdrawable switchboards. For all normal operations with the doors closed, operators can depend on passive arc fault protection. When working with doors open, the arc detection system would provide active arc fault protection.
  • On old installations with HV fuses, protection could take several seconds or even minutes to clear a high impedance arcing fault, resulting in catastrophic damage to the incoming section of LV switchgear. Installing this active protection on the LV switchgear incoming terminals, the arc would be extinguished in under 5 ms. The HV fuse would operate cleanly on a short­circuit to clear the fault.

Fig. 3: Arc monitor with detectors.

Conclusion

Of course, there are other active arc fault protection options such as smoke and heat detectors and pressure sensors. The smoke and heat detection systems may not be fast enough to provide personnel with protection. Pressure sensors are a good option as the pressure built up as a result of an arc would be detected in 5 to 10 ms, and is often used in MV and HV applications. They must be tested to the guidelines in IEC61641 to ensure any of these arc protection systems will protect workers for arc burn injuries.

Active protection devices, in comparison to passive protection, are more susceptible to failure owing to the additional electromechanical and electronic devices. The proven best practice is to prevent the fault arcs from occurring in the first place, eliminate the need to work live and then in the unlikely event of an mternal arc, contain the arc to that compartment.

Acknowledgement

This article was originally published in Industrial Electrix and is republished here with permission.

Contact Avi Ramdhin, ABB, Tel 010 202-5000, avishkar.ramdhin@za.abb.com

Related Articles

  • Gas detector for multiple applications
  • South African coal faces bleak outlook
  • South African coal exports outlook: Approaching long-term decline
  • New variable speed drives
  • Measure overall and bearing vibration simultaneously