Current Limiting Fuse and Arc Flash

Question: Mike, I am wondering how many folks are finding that they have some pretty nasty areas at their main service entrances when looking at the arc flash conditions present. For years I pressed for current limiting fuses in main switches where ever possible. Now that we have had an update to our fault current study and an arc flash report prepared we are seeing that these nice current limiting fuses really are not doing that much for arc flash. If I'm understanding the calculations used for arc flash it is not the available fault current but some "reasonable" value, like maybe 75% used ...

Well we are seeing that when this value is used you push the fuse out of the current limiting range in a lot of instances. The 2000 amp Class L fuse I expect to clear in less than a cycle is taking 1 second to clear.

This makes the PPE requirements excessive and in some instances we get a "stay out period, do not work on this equipment while energized" determination. Anyhow, just wondering if you've seen/heard similar tales...

Thanks again for sharing your knowledge of electrical systems...

Jim Koryta

Answer from Cooper Bussman: Mike, let me try to help answer James Koryta's question. The arc-flash energy associated with a 2000 ampere overcurrent device is horrific, whether that device is a current-limiting fuse or a circuit breaker. Actually, the energy level let-through for all devices above 800 or 1200 amperes becomes a real concern. It is a matter of the length of time that it takes for the fuse or circuit breaker to open. For current-limiting fuses or current-limiting circuit breakers, the higher the available short-circuit current, the higher the available arcing current, the greater chance that the current-limiting device will be in its current-limiting range.

Once the device is in its current-limiting range, the arc-flash energy is drastically reduced. But as James points out, the available arcing current is not always high enough to cause the current-limiting device to be in its current-limiting range. For example the arc-flash energy let through by a 1200 ampere Class L fuse with 50,000 (or greater) short-circuit amperes available will be .39 calories per cm squared or lower. However, with only 30,000 short-circuit amperes available, the arc-flash energy is 5.16 calories per cm squared. Believe me, a 5 calorie per cm squared event is one that you would never forget, if you lived through it.

Is there an advantage to using current limitation? Absolutely. What would the arc-flash energy be if a short-time delay for 30 cycles were utilized to obtain selective coordination? At 30,000 short circuit amperes available, the arc-flash energy jumps from 5.16 calories to 58.01 calories. Simple arc-flash calculator .

Does a current-limiting device always protect the worker so that they do not have to wear heavy layers of PPE? Absolutely not!

So what else can be done for these larger circuits?
• Require insulated bus where available to help prevent the arcing fault from ever starting.
• Utilize a shunt-trip on switches and fused circuit breakers above 800 amperes that opens the switch when the first fuse opens.
• Utilize zone-selective interlocking where available.
• Utilize instantaneous trips rather than short-time delay.
• Utilize smaller overcurrent devices whenever possible. For example divide a load into five 800 ampere devices instead of one, 4000 ampere device.
• Keep feeder runs as short as possible to keep the available short-circuit current as high as possible, so as to be in the current limiting range of the current-limiting fuse or current-limiting circuit breaker.
• Specify arc-resistant switchgear (actually arc-diverting switchgear) where available. See Powell
• Utilize vacuum interrupter equipment (For example see Cooper Power Vacuum Fault Interrupter ) on the primary side of a substation transformer that can be triggered by an arcing event on the secondary side of the transformer. Such equipment can open in about 3 cycles after receiving a trip signal. The trigger might be from a device such as the one below.
• Utilize relays that look for both an overcurrent condition and the light of the arc, and then send a signal to open the circuit. ( For example see ABB )
• Utilize relays that change the settings from short-time delay to instantaneous whenever a person is working on or near the switchgear, and then changes back to short-time delay when the person leaves. ( See Siemens on page 7 .)

I hope that this has been helpful. It would be great if this would have been a total solution, but unfortunately, it's not. It is refreshing to see that so many
manufacturers are working on solutions ( see our safety material ) and that so many users are putting the safety programs in place to protect their employees, as best as possible, from these hazards. Thanks for the opportunity to respond.

Vince Saporita
Vice President Technical Marketing and Services
Cooper Bussmann