|> So how would you wire up a panel if the utility says that your available |> fault current is, for example, 15kA, when individual branch breakers are |> only rated to 10 kA? Two major product lines (Eaton/Cutler-Hammer CH and |> Square-D HOMELINE) have no branch breakers available above 10 kA. Even |> in other product lines, higher rated branch breakers are available only |> lesser choices. | | You select a panel and branch circuit breakers with the appropriate | rating.
You mean a $20,000 industrial panel?
My point is, the choices are rather slim above 10 kA. Not everyone needs to worry about 10 kA, yet, but it seems more and more are.
The CH line has MCB panels rated to 100 kA, but individual breakers do not go above 10 kA.
The QO line has MCB panels rated to 22 kA, and invididual breakers rated at
10 kA and almost all available for 22 kA. A few go above 22 kA. Only the monster 400 A panel could have a substituted L-frame breaker for very high interrupting capacity as much as 100 kA.
The HOM line is rather skimpy.
the BR line reverses what the CH line has ... panels don't go so high but it has branch breakers up to 42 kA.
The ideal would have both branch breakers and main breakers rated at various levels up to what is marketable to residential and small commercial. That should, IMHO, be around the 10 kA, 22 kA, and 42 kA levels.
|> Will people have to be demanding lower fault currents from the utilities, |> which otherwise seem to be increasing them as they deal with the increasing |> demand? Energy-efficiency is also contributing, by pushing down temperature |> rise ratings of transformers, which have some higher fault currents due to |> larger conductor sizes. | | Interesting note: Our local utility (and probably many others) has a | standard practice that | requires fault currents to residential services of 200 and 400 A ratings | to be kept under 10 kA.
Interesting. So keep the transformers from getting too large, and just have more of them?
| For commercial services, its a whole different ball game. In fact, this | is somewhat of an issue with another local utilities' network secondary | systems. As loads grow, they increase the secondary buss capacities and | add more transformer capacities feeding them. But many of the customers | connected years ago were quoted lower fault duty ratings than what are | available now. So, whose responsible for upgrading the service equipment | or installing C/L fuses?
And I assume they can't segment the network?
|> Should one install a transformer to reduce the available fault current |> (with a high interrupting main breaker on the primary side)? |> |> |> Though it has never happened to me, I've heard of cases where people have |> |> had short circuits that resulted in both the branch breaker and the main |> |> breaker tripping. While that can be annoying, obviously you don't want |> |> the main breaker to wait for too long before "deciding" that the branch |> |> breaker isn't able to interrupt the fault. |> | |> | This is a case of poor breaker coordination. It can result from the use |> | of breakers not rated for the available fault current. For example, the |> | branch breaker, with a lower overcurrent rating, would be expected to |> | act sooner for any given fault current. However, if it is incapable of |> | breaking too high a current quickly enough (or at all). Even as the |> | branch breaker is opening, sufficient current will pass, and the main |> | breaker will sense it, so that the main will reach its trip point. The |> | end result is that both breakers operate. |> |> Could it be an overly fast main breaker? | | You mean one that's out of spec? Possibly. But poor protection | coordination design is more likely. If the main is spec'd to be quick, | the branch breakers should still be quicker.
How quick, or slow, should they be under peak fault conditions?
|> I would think that a really serious fault approaching the available fault |> current would put things down in the flat line of the interruption curve, |> and both breakers should be trying within a fraction of a cycle, and at |> least the main should succeed by at least the next zero crossing. |> |> If given an available fault current in excess of available branch breakers, |> wouldn't you at least want your main breaker to be one that interrupts as |> rapidly as possible to minimize the downstream damage from such a fault? |> Or would it, because it trips too often in non-damaging cases, lead people |> to think that in a scenario where there is damage, to just reset it without |> investigating causes and damages? | | At high levels of fault current, all of the breakers will be quick | enough. Having the main beat a branch breaker is at minimum a nuisance | and will make troubleshooting much more difficult.
Which hopefully doesn't happen at home too often. It could be a mess in a commercial situation where fault currents could be larger and more loss incurred when a main breaker opens.