Good news doesn’t usually make the news: stay safe with passive arc protection in medium-voltage switchgear
Let’s stay safe and out of the news
Let’s stay out of the news. When you turn on the TV or listen to a news broadcast, it seems most of it is bad news. You might question why that is, and maybe it is because most of life is good. This makes what is out of the ordinary newsworthy. In our line of business there are always potential hazards, as there are high levels of energy and voltages involved. Working with medium-voltage (MV) switchgear, we know that internal arc flash accidents are rare, but when one happens, the consequences are severe. It is of utmost importance to pay attention to arc resistance and the safety of personnel – so as not to create another devastating news item.
The key to personnel safety when working with any MV equipment is appropriate training. Operating and maintaining MV equipment, such as switchgear, should only be performed by authorized and trained personnel, as in the case of an arc flash, the consequences can be lethal or cause severe injury.
An arc flash is the most unwanted phenomenon in electrical systems. It is a type of electrical explosion or discharge that occurs when electric current leaves its intended path and travels through the air between conductors or to ground. The most common causes of arc flashes are human mistakes, insulation failures, and over-stress of components, such as when a circuit-breaker fails during a switch operation.
The energy released during an arc flash in an 11kV switchgear can be up to 300MW, which is equivalent to the amount of energy needed to start six space shuttles. The temperature may rise up to 20 000ºC, which is five times more than the surface of the sun. In the chart below, we can see that after 100ms the cables will ignite, after 150ms cooper starts to burn and after 200ms metal sheets starts to burn.
On top of ensuring that the personnel is properly trained and skilled for the task, the switchgear must be type-tested and classified A. There are other ways to further manage and reduce the hazards of arc flash accidents, starting out with selecting the correct internal arc current and duration rating for the application, and defining the correct classification for access to the enclosure.
The IEC 62271-200 Edition 2 standard lists three categories for restricting access to the enclosure:
• A – restricted access to authorized personnel only
• B – unrestricted access
• C – restricted by installation out of reach
Categories A and B are to be marked with additional letters F(front), R(rear), L(lateral), to indicate the sides of the switchgear that are arc resistant. The classification of switchgear have to be clearly indicated on the switchgear label, including arc withstand current in kA and duration in seconds.
Last, but not least, choosing the correct gas duct system. When there is an internal arc in the switchgear, hot gases and metal debris must be evacuated in a safe manner. To do this, each switchgear is to be equipped with a gas duct system. There are two approaches: either evacuation outside of the building or safe release inside the switchgear room.
For evacuation outside the building, there is a gas duct with an outlet. This type of solution is very effective for smaller switchgear rooms, such as containers, as it does not set particular size requirements. This approach calls for additional civil works for preparing holes in the wall – and also additional safe space is required outside of the building – and it must be ensured that this space is fenced.
Also the most suitable type of ending (grid, flap or hatch) to the gas duct system needs to be defined.
The second approach is to release hot gas inside the switchgear room, which requires a top chimney solution. With this approach, no additional civil works are required, but extra space above the switchgear has to be available. This space has to be clear of any material above and it is highly recommended that a calculation of pressure increase inside the switchgear room is made, especially if the switchgear room is small.
As we have seen there are measures that can be taken to make working with MV switchgear safe right from the start. Let’s stay safe and out of the news and ensure that we have nothing to report.