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Medium Voltage Disconnect Rating - Disagreement with Boss

Medium Voltage Disconnect Rating - Disagreement with Boss

Medium Voltage Disconnect Rating - Disagreement with Boss

I am an electrical engineer for a North American municipal electric utility. I am having a disagreement with my boss over the rating of a new medium voltage disconnect.

Project details:
- New disconnect to be installed in existing 35 kV open-air rack.
- New disconnect will be used to open transition (break before make) load between 2 separate out-of-phase sources.

Source 1 (normal source)
38 kA sym available fault current (3PH / SLG)

Source 2 (backup source)
15 kA sym available fault current (3PH / SLG)

- New disconnect will be open point between Source 1 and Source 2
- After Source 1 is de-energized via circuit breakers, new disconnect will be closed to move load to Source 2 (dead bus transition)

Disagreement with boss is over the short time withstand rating (3 second) of new disconnect.

My take is that since new disconnect will be connected to Source 1 in the open position - it still needs to be rated for Source 1 (i.e. 38 kA sym) since there may be animal / mylar balloon contact between energized portion of new disconnect and ground.

My boss believes new disconnect only needs to be rated for Source 2 (i.e. 15 kA sym) since it will never be in the closed position while connected to Source 1. New disconnect will only be in the closed position when connected to Source 2.

Which is the correct approach?

RE: Medium Voltage Disconnect Rating - Disagreement with Boss

Agree with you...


RE: Medium Voltage Disconnect Rating - Disagreement with Boss

I vote with your boss. The SC rating is based on current flowing through the switch. If the switch has a fault to ground when open, then the current doesn't flow through the switch. The switch may be damaged by the short circuit; it is not rated for a fault on the switch.

RE: Medium Voltage Disconnect Rating - Disagreement with Boss

The switch only needs to be rated for currents that can flow through the closed switch.

If it is possible for someone to come along later and change the operating sequence, it may be better to install the higher rated switch as a precaution.

RE: Medium Voltage Disconnect Rating - Disagreement with Boss

Agree with bacon4life. Just because something was designed to operate one way does NOT mean it will ONLY be operated that way. Lots of experience over the years proves there is still no shortage of "uninformed" people. Although you only have to size it for the way it's SUPPOSED to work (15 kA), my personal approach would be to size for the greater current (38kA) Just.In.Case.Things.Change.

Converting energy to motion for more than half a century

RE: Medium Voltage Disconnect Rating - Disagreement with Boss

I think you hit the nail on the head. I think that technically 15kA would be the minimum required here.

However, you never know how things might change in the future. Or how someone who doesn't understand the limitations you impose right now may choose to operate the system when you're gone.

I would look at the cost differential between the 38kA and 15kA and make a judgment based on cost and the potential cost if it needs to be changed in the future. If we're talking $1000 on a $10000 piece of equipment, it may be well worth it to bite the bullet now as you may spend most of that just changing it out if you need to in the future.

But this is where "engineering judgment" comes in. At what point is it "good enough" to be safe and fit for purpose while also maintaining economy?

RE: Medium Voltage Disconnect Rating - Disagreement with Boss

Συμφωνώ με το bacon4life

RE: Medium Voltage Disconnect Rating - Disagreement with Boss

Well, the close and latch rating is important too because if someone inadvertently closes the switch into a fault that was not correctly cleared downstream? I've seen cutouts and other fused devices pushed beyond their short circuit ratings. The question is a matter of are you OK with things failing abruptly and what is the percentage that would actually occur? Sometimes that chance is very low. It is always application specific too. If the switch is already closed, then its thru fault rating may be different than its close and latch rating (momentary). Conservative approach would say apply the 40kA rated switches for your application. But that is more costly.

RE: Medium Voltage Disconnect Rating - Disagreement with Boss

Another to thing to consider is if your ratings on the one line are considered with a tied bus or not. I feel like those fault current ratings would be with a tied circuit as I think your system is generally 25kA at 34.5kV near the substation (RS) without lines tied based on previous information I have encountered. I think I know which utility you are with at least.. Fairly strong system as my memory serves needing to use 25kA rated gear and fusible devices 17.5kA..

With that being said, most utilities do not consider the impacts of temporarily tied lines in the withstand ratings of the equipment, except at the substation where the breakers are typically rated for a temporary tied bus (if you normally operate the circuit with open station bus ties between each transformer, that is). In the worst case scenario, at least the feeder breakers can handle interrupting.. Its another one of those cost balances as yes you could boiler-plate say we have 40kA available and we must use that. But it really depends on what is going on with the rest of the circuit if that is a typical configuration or not. You also have to think about the odds that a fault will reach those magnitudes. There is always some level of fault impedance, so again, it is a trade off of cost and benefit and if that provides you with an acceptable level of system protection. Based on where I think you are at as well, there are other recent things to consider, such as fire danger of failed devices. Also another elephant in the room is use of gasses in switchgear, that may become an issue for you given your system and one that is unfortunately political. I'm sure it has been discussed and I know gear manufacturers are trying to work up solutions for utilities with strong available fault currents.

My work specifically is with industrial and commercial users, so the way I approach things is a little different from a utility. Specifically, I'm applying maybe devices in the hundereds or maybe thousands. Utilities are applying hundreds of thousands of devices. So changing specs means a heck of a lot in terms of costs if it isn't necessarily warranted. But for me, going to a higher rated device won't be the end of the world, especially in an area where you do not have the same skilled operators as utilities have.

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