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Switching sequence for powering down and up

Switching sequence for powering down and up

(OP)
Hi, I recently joined a plant where the E/I super has been performing switching the opposite of the way I was taught and I am looking for some references on switching order. In general I've always dropped loads first and opened from bottom up when de-energizing. For energizing, I've always started at the top of the system and worked down. The E/I super at our plant has been doing just the opposite, opening the main breaker first to de-energize, and when closing back in he will close all the downstream breakers first and then close in the main breaker.

His rational is that they've always done it that way and that doing it my way will take longer. My rational is that it is more stressful on the main breaker to close it with a load on it, there is potential for tripping due to inrush, and if there is a problem downstream it will be more difficult to find because the main breaker has tripped off.

Any opinions or reference material on the subject? I'm tying to find a technical reference to back up my point of view.

Thanks

RE: Switching sequence for powering down and up

Hey EEJoe,

As an operator from way back, I was taught to switch the same way you were; although I cannot quote chapter and verse from any reference material on the subject, I have come to understand the way both you and I were taught to switch to be "good operating practice," something you may not find documented anywhere but that is generally accepted industry-wide.

For what it's worth I much prefer your rationale to that of your new-to-you E/I supervisor/superintendent, especially your last point, as any faults or defects that have developed while equipment is out of service will be readily detected during restoration switching, allowing the balance of the equipment to be returned to service without undue delay.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Switching sequence for powering down and up

I'm not aware of any technical reference; and I'm not sure it particularly matters on deenergizing. But restoration should always occur bit-by-bit. During the outage everything* needs to be opened; then the main is closed to pickup the bus of the main gear; then those breakers are closed out to the next level; repeat level by level as necessary.

*My world is all big stuff and that restoration sequence might get to 7 or 8 breakers for all but the largest outages. I don't know what level you're working at, but there can be stuff too small to worry about. I'd not bother opening all the circuit breakers in every branch panelboard; there's some amount of load that can be picked up in one chunk without much concerned.

RE: Switching sequence for powering down and up

I have seen several installations where the main breaker could not support block loading of the entire load.
In such a case the main breaker MUST be closed first.
Many smaller services and a lot of larger services are capable of energizing the entire load by the main breaker.
Best practice is "Main first and sub feeders next".

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Switching sequence for powering down and up

On the utility scale during a transmission reclose event, more than 100 MW might get dropped and picked up in a single block. If the outage is going to last long enough to cause cold load pickup, dispatchers would typically all breakers at the substation to clear the bus, then close the HV breaker, and finally start closing individual feeders. For short duration outages, dispatchers have discretion as to whether to restore the transmission line without first clearing all loads.

RE: Switching sequence for powering down and up

(OP)
Thanks for the responses

RE: Switching sequence for powering down and up

In all honesty its more experience and understanding inrush, pickup, and the like. But if the system can handle it (inrush, voltage drop, ect), there is nothing wrong in theory with turning everything on all at once. There are radial 69kv sub-transmission lines handling over 40MVA of load that will trip and reclose in seconds during a storm. But after a lengthy outage, especially during the summer, thermostats that were cycling relative to one another in an air conditioned building are all now calling for cooling in every building, so in that case distribution circuits are picked up one at a time, sometimes each individual circuit in sectionalized segments. Otherwise, the load would be to great and immediately trip out over current elements on the transformer or possibly feeders.


When switching from utility to generator during an outage, typically the biggest motor load is thrown on after all the life safety circuits have transferred in 10 seconds or less with smaller loads being transferred in session (larger to smaller).

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