Your take on Arc Flash labeling 2

[SJBatTCE]

When you have a panel feed directly from an unprotected source (i.e., large transformer secondary) you will generally have a high arc-flash hazard at the incoming lugs of the panel. On an installation such as an I-line (or similar construction), will the selection of the main breaker settings reduce the HRC for the panel or does the entire panel require labeling for the hazard present on the main breaker's source terminals?

 

[dpc]

For a group-mounted panelboard or switchboard, you should always base the arc-flash calculation on the next upstream device that is remote from the panelboard or switchboard in question. It has to be assumed that a fault inside the enclosure could involve the line side terminals of the main breaker.

For switchboards and switchgear with individually-mounted breakers, you might make a case for taking credit for the main breaker for a fault in another section. There's no real test basis for that - just interpretation and common sense (or maybe wishful thinking).

But for group-mounted equipment, I don't believe there is any real debate. Just pretend the main breaker is not there.

 

[lansford]

The issues is that if you have a main CB in a panel and you can contact the line side (which is usually the case) then you would ignore the main CB as the previouus psoter noted.
JIM

 

[SJBatTCE]

I can agree with that logic. But I also think that with panels that provide barriers over the main breaker and incoming feed that require additional disassembly beyond what is required to access the buswork should substantially reduce the risk of accidental contact with the higher available energy conductors. If that can't be taken into consideration, then having the panel assembled when operating a breaker can't be considered a lower level hazard.

 

[SJBatTCE]

What would be reasonable to reduce the hazard for this panel directly off of the transformer secondary? 2000A fused disconnects are rather expensive and you've transferred the HRC to another device.

 

[dpc]

The hazard is not just accidental contact with the line side of the main breaker. Even if the arc is initiated downstream of the main breaker, the conductive plasma created by the arc can very quickly engulf the line side terminals and cause an arc there as well.

You might be able to make a case for taking credit for the main breaker if there was a very substantial and complete enclosure/barrier around the main breaker. But I never seen one in a panelboard or group-mounted switchboard.

One option might be transfer trip: addition of current transformer and relays to allow the upstream primary protection to be shunt-tripped when a fault is detected by the relays.

 

[jraef]

The hazard is not just accidental contact with the line side of the main breaker. Even if the arc is initiated downstream of the main breaker, the conductive plasma created by the arc can very quickly engulf the line side terminals and cause an arc there as well.

dpc is absolutely correct about this. Arc-Flash has little or nothing to do with accidental contact, it's all about plasma. Only if the main were in a completely separate enclosure from the rest of the equipment in the panel and the cables going in and out of it were sealed would there be a possibility of reducing the PPE level of a panelboard.

But in a general way, your idea is a sound one at another level. The newest Insulated Case Power Breakers are coming with selectable trip settings so that, for example, you can remotely lower the trip thresholds in order to reduce the Arc-Flash hazard when someone enters an equipment room. While someone is there, the risk of a nuisance trip would be increased, but it often allows testing and simple maintenance or record keeping with a lower level of PPE, i.e. from PPE-4 down to PPE-3.

However for something like an I-Line panel, I doubt there is anything you would be able to do that would lower the risk substantially enough anyway. Low voltage low current systems like where a I-Line would be used already will have relatively low Arc-Flash energy levels to start with. In other words if you are already at a level low enough to allow PPE-2, there is nothing you are going to be able to set on a breaker to get it down to Level 1.

 

[SJBatTCE]

Taking into account the plasma stuff, I see your point. thanks for taking the time to help me reconcile my own conflicting opinions about this.