What happens when you blow a fuse really hard?

But for a parallel run the wire is still backfed from the load end (unless there are cable limiters on both ends). The utility is protected but I would think the customer would still have a failure.

And opening one wire puts the load on the other parallel runs (but what better option).

Not obvious to me why there is more advantage on parallel runs than single unless because the wires are usually larger.

----------------- The building where the service was replaced had burned down so only the basement and 1st floor slab remained. That was used for parking. The original 208/120 service in the back corner remained - 6 parallel runs. Someone had the bright idea of storing snow melting salt on top of the switchgear. In the resulting burndown some of the wires burned back into the service conduits. At least a couple of the wires welded to the conduit - the utility broke a come-along trying to pull one. Some of the limiters didn't blow - must have burned free. But the utility was protected.

Its possible to install current limiting fuses ahead of service equipment that would not, by itself, withstand an increase in available fault duty. But, from what I've seen, utilities rarely know when their upgrades will push customer equipment beyond its rating.

Utilities I've worked with do not keep track of customer facility specifications. If a knowledgeable customer gets wind of an upgrade, they may do a coordination study and install CL fuses (rather than upgrading the service equipment). But very few customers are knowledgeable and most, if warned, wouldn't understand the consequences of ignoring the heads up.

On Fri, 28 Mar 2008 11:04:35 -0700 Paul Hovnanian P.E. wrote: | Ben Miller wrote: |> |> Paul Hovnanian P.E. wrote: |> |> >

|> > Now, try to teach this to a utility that serves its customers from a |> > networked secondary system or with primary service. Back when a |> > customer originally requested service, the fault current available on |> > a 480V service might have been around 50 kA. Their service equipment |> > was sized to interrupt this level of fault current. |> >

|> > As the years go by and more capacity is added, the fault current |> > available can go up to 100 or 150 kA. Does anyone bother to notify the |> > customers? Not as far as I've seen. |> >

|> > The problem is either caught when someone does a major remodel, |> > requiring the service equipment design to be revisited, or when the |> > service equipment (and sometimes the building) fails catastrophically. |> |> I suspect this is a much bigger problem than people realize. Even some |> residential areas have increased as more & bigger homes are added. Your |> house might be on the same pole pig, but the primary side could have much |> more available SCC than it used to. Even if the utility informed customers, |> how many companies or homeowners are going to replace entire service |> panels/switchboards? As you said, it will only get done when it is upgraded |> or redesigned for some other reason or when it fails. | | Back when I was a distribution engineer, my standard practice was to | calculate the secondary fault current available based on a distribution | transformer located just outside our largest standard substation. Even | if the customer was 20 miles down a farm road. One never knows if the | next big substation will go in just across the street from him. | | But this wasn't standard practice across the company. And for larger | commercial/industrial customers, they need more information than just a | maximum fault current level. Any significant change to the system needs | to be looked at. But when I'd call a customer about a system upgrade (a | 12 kV to 34.5 kV cutover for example) and suggest they forward it on to | an engineer, I was usually met with a blank stare. Even some primary | service customers, who are supposed to have "qualified" personnel | operate and maintain their systems had no one on call. Some of our | linemen made good money on the side, as they were the only people in | town with the tools and know-how to do H.V. maintenance.

One of those MBA type people, had they known what you did, might have been able to accuse you of not saving the company as much money as you could. Never mind the liabilities you avoided ... or even the higher future costs.

As for H.V. maintenance ... I watched a substation near here being upgraded for a couple months period along my daily drive route to work. It appears all the work was contracted out. Only a couple times did I see AEP trucks there. The rest of the time was one or more of about 3 different private contracting companies. I suppose the industrial customers could have used them as well.

I sure don't want the hassles of having 12 kV to 34.5 kV coming in to my house :-) I'll stick with the "under 600 volt" service as long as the available fault current stays under 22kA (the rating on my panel main). I'm on a 100 kVA pad with 2 or 3 other houses (it's how to tell how the two pads in the neighborhood are exactly devided up without calling in the guys that come spray paint red lines all over the yards). It would have to be under 2% to exceed my ratings, assuming infinite distribution fault current and superconductor underground service. I do get noticeable light dimming all over the house when my A/C compressor starts, lately, so I've probably got plenty of impedance.

Yes, you need them at both ends in order to isolate the cable.

Exactly. Of course, without some type of fault detector, nobody is aware that a fault occured or that the service is not running on the full conductor ampacity, so if it is heavily loaded it is likely to fail over the long term anyhow.

They protect the cable in either case. However, with parallel conductors they add the function of isolating the one faulted conductor, and allowing continued operation. I have never seen them on a single service conductor.

Here is the Bussman info:

Larger? The individual conductors of a parallel run would be smaller than a single conductor.

That is one reason to use cable limiters. For a single conductor (per phase), one can protect them with a transformer primary side fuse. On the other hand, each conductor of a paralleled group is significantly smaller than the total service rating.

In the event than one of the group opens, the remaining conductors will carry more load current, even though the upstream protection will 'see' nothing unusual. Once the load current exceeds the cable limiters rating, it will open. As the load current shifts to the remaining cables, their limiters will open as well.

I did not clearly state what I mean. The wire for parallel runs is likely to be larger than a different run that was just single. The idea was parallel runs are near the max size wire that is economical, but a single run is not because the loads vary from small to large.

If a transformer feeds only one set of wires, the transformer primary fuse may provide short circuit protection. Utility transformers often feed multiple services.

The one place I saw cable limiters was in a downtown utility transformer vault. The 800A service I was working on was almost trivial compared to the transformer rating. But if there was a fault on my wires why couldn't it propagate into the vault and cause some real excitement? Why wouldn't cable limiters (utility end only) be useful on my single set of conductors?

Ben, and the Buss data sheets he provided [thanks], say cable limiters are for short circuit protection. The curves on the data sheets do not go out to a long enough time to determine what kind of overload protection the cable limiters provide.

(The data sheets show those limiters are current limiting.)