Arc flash protection: combining containment and mitigation
Credit to Author: Tarek Boumegoura| Date: Wed, 08 Aug 2018 13:00:27 +0000
The Occupational Health & Safety Association (OSHA) have reported that arc flashes result in over 7,000 burn injuries per year in the US, with over 400 people dying as a result. That’s an average of 1 to 2 arc-flash related deaths per day, a disturbing statistic.
In my last post in this blog series, I discussed the cause of arc fault and arc flash events, the serious risks to electrical safety they pose to people and equipment, and an overview of protection methods. Without any protection, an arc fault can be maintained for a longer time. This, in turn, means that destruction, including electrical fire, will be propagated outside the electrical switchgear. There are many methods to reduce the arc occurrence, reduce its propagation, and mitigate its impact.
These solutions can be designed during construction of the switchgear or added later in the retrofit process. Now, let’s look at the two approaches that can be used: active and passive arc protection.
Passive arc flash protection
These two types of protection are part of the mechanical switchgear construction, designed to reduce the propagation of arcs.
- Arc Containment: This method was mentioned in my last post. It uses a reinforced panel to contain the shock and avoid fault propagation. The switchgear should be type tested to IEC 60298. According to this standard it is possible to specify an internal arc rating with fault times of 100, 500 or 1,000 ms. The switchgear is designed to remain intact in the event of an internal arc, at least for the specified internal arc rating time. However, it will not prevent internal damage. Arc-resistant switchgear will not be efficient if the switchgear door is open, which is typical during maintenance or commissioning.
- Arc Isolation: This method isolates all conductive parts. Insulated equipment is used to isolate bus bars, cables, and connectors. This avoids any arc ignition on any conductive element. When choosing this method it’s best to anticipate the panel design to include that level of protection. This solution will still be efficient when the panel door is open during maintenance. However, it’s very difficult to equip existing panels with this solution.
Active arc flash protection
Also mentioned in my last post, this method uses optical detection of an arc flash ignition. This allows early detection of the fault and, in turn, activates circuit breaker protection to extinguish the fault. For the most efficient protection, sensors should be deployed in all zones. The benefits of this method are that protection can be activated as quickly as 1 ms, and is responsive even if the switchgear door is open. And with professional design, it is possible to deploy selectivity to avoid a global shutdown.
The above three solutions could be considered as three barriers:
- Arc isolation will reduce fault ignition risk by switchgear design.
- Arc flash protection detects faults and acts on protections to reduce duration and mitigate impact.
- Arc containment will limit the propagation of damage if an arc shock occurs.
For critical sites, arc isolation will avoid the arc ignition and reduce the risk of arc fault. However, human errors during maintenance can still occur, as can faults due to animal damage. In this case, the active flash protection will be an advantage. The protection will isolate the damaged circuit and allow for recovery in a shorter time.
Arc flash resistance should be mandatory in medium voltage switchgear, as this will reduce the damage propagation. The maintenance should only be needed on the damaged switchgear. Of course, the consequence of human errors and animals will not be protected against. The active protection should be used to reduce the damage and protect the installation.
Few low voltage panels are arc resistant and will typically not have any active arc protection. Depending on the incident energy, it’s a great advantage to install active protection on such panels.
Finally, existing switchgear cannot be transformed to an isolated design afterwards. Dirt and dust decreases the insulation degree in the switchgear increasing the potential risk for the arc flash fault. To protect against the arc flash fault consequences, the active optical protection will be the easiest, most cost-efficient, and faster solution to deploy.
For additional information, read more electrical safety articles here on the SE Blog, visit our arc flash solutions page, or download our white paper “Arc Flash Mitigation”.
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