It becomes a hazard when a large enough energy is released in a short enough duration of time.
➤ Arc flash is most likely to occur during maintenance activities, when a person might accidentally short two points at different potentials.
⇒IEEE 1584 – Guide for performing arc flash hazard calculations and calculation of incident energy.
➠ Arc flash incident energy at a point is mainly dependent on three things – fault current, fault duration and distance of that point from the point of fault.
➠ To minimize fault current, we can have fuses. Protection settings can be set to low to minimize the tripping time thus limiting the fault duration (although this may result in false trips).
⇒IEEE 1584 – Guide for performing arc flash hazard calculations and calculation of incident energy.
- Category 0: 0 - 1.2 calories/cm2
- Category 1: 1.2 - 4 calories/cm2
- Category 2: 4 - 8 calories/cm2
- Category 3: 8 – 25 calories/cm2
- Category 4: 25 - 40 calories/cm2
➠ Arc flash incident energy at a point is mainly dependent on three things – fault current, fault duration and distance of that point from the point of fault.
➠ To minimize fault current, we can have fuses. Protection settings can be set to low to minimize the tripping time thus limiting the fault duration (although this may result in false trips).
➠ We can reduce the incident energy (say at a point outside a MCC) by providing ‘other means’ for the arc flash energy to get dissipated. So, if we provide a vent on top of a MCC, the arc flash energy has more space to dissipate energy than just going straight out. This would reduce the arc flash boundary (defined as the point at which there is a 50% chance for an unprotected person to get 2nd degree burns).
The object of this exercise is to raise your awareness of arc flash hazard.
The object of this exercise is to raise your awareness of arc flash hazard.
- If you leave equipment alone it is unlikely to arc, and due to probability, if it does, it is most likely that anybody is there.
- You are right, arc flash is more likely to happen when individuals are there, because of the very activities they are performing, be it because they cause a short directly, or because there is a faulty component which is disturbed by their activities.
- First line of defence is not to work on or near live equipment
- Second line of defence is in the design as you can never design out the hazard, only reduce it.
- Last line of defence is PPE.
- I was at an ABB Seminar the last couple of days and I came across this:
- Ultra-Fast Earthing Switch (UFES) – It is a sacrificial device to protect equipment in the event of an arc fault. What this essentially does is convert the arc fault into a “bolted” fault, by connecting the phase to earth in 2ms, so that fault current flows through this device instead of the actual fault location. It has a micro-gas generator (which are also used to inflate air bags in vehicles), which ruptures, resulting in the moving contact travelling like a bullet to bond with the fixed contact and created a bolted fault.
➤The device costs around $10k and the replacement cost is somewhere around $4K. Probably a lot cheaper than having to replace the whole switchgear after an arc fault occurs.
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