LVSG (Low Voltage Swichgear) 1

[yamyam]

Hello guys,

Sorry for this newbie question. I'm just wondering, when will you know that your design needs a Low Voltage Switchgear, I mean are there any minimum requirements for it (eg. Voltage, Current, kVA ratings, number of panelboards) I guess you can understand me by now.

I just want to learn and hear other's knowledge for this topic.

Thanks guys!

 

[davidbeach]

If switchgear solves problems, use switchgear. If switchgear creates problems, don't use switchgear. Where it crosses from one to the other can depend on many factors, including owner preference and engineering experience. Big projects are probably going to have switchgear and small projects probably won't, but there's certainly no bright line that divides one from the other.

 

[waross]

You design panels or sub panels sufficient to carry the load and anticipated future loads. You feed these from a main panel. When the total anticipated load is greater than can be supplied from a single incoming service panel you consider adding switchgear to supply main and sub panels.

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

 

[yamyam]

So when can you say that you need a switchgear? After your project has a greater load than the anticipated load? Or can you pre-design it? I'm very sorry for asking newbie questions guys. Appreciate the help and knowledge.

 

[davidbeach]

There is no single criteria. But when you need a board to distribute 2000 Amps or more you may find switchgear advantageous. There are high ampacity panelboards and they will take less space, but that less space means more fighting with the cables to make all the connections.

 

[jraef]

Actually, it often involves fault tolerance, not just for the individual circuits and gear, but for the facility as a whole. Using ANSI concepts, OCPDs in SwitchGEAR are at the top of the chain. The breakers are set and intended to hold in longer to allow for faults to be cleared by SwitchBOARD and Panelboard OCPDs at levels closer to the overall fault, but be ready to trip if that doesn't happen. So in that longer cycle time, the gear holding those OCPDs will need to be stronger to withstand the added mechanicsl stress that entails, and when that Breaker does finally clear the fault, contain the damage it might cause to prevent it from taking down the entire facility. That is done by encasing the SwitchGEAR breaker into a "cell" that prevents that collateral damage.

If on the other hand if your facility is such that this wouldn't apply, a Switchboard or Panelboard might be all you need. That generally depends on the size of the service and/or the available fault current from the source and/or the nature of the business. For example if your process cannot continue on with any part of it being off line anyway, the level of sectionalization afforded by using switchgear may be pointless and you may as well use Switchboards or Panelboards anyway.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[yamyam]

Thank you guys for responding. I'm just kinda new in the design world. I hope you understand.

@jraef: How can you determine that you need a "fault tolerance", I mean how can you specifically say that, this project needs it and this other project doesn't.

@davidbeach: Technically, you need a switchgear for let's say, "spacing purposes"? Sorry for being a newbie.

 

[davidbeach]

Panelboards mount on the wall and don't require (or allow) rear access. Switchgear puts the cable connections at the rear, thus requiring both front and rear access. Lots of breakers in a panelboard, only four per section in switchgear. jreaf's concerns about fault duty is valid but as I recall it was possible get similar ratings in panelboards as in switchgear; much more room to lace the cables together in switchgear. But I haven't looked at that type of equipment in over a decade.

 

[jraef]

By fault tolerance in this sense, I was meaning how your entire facility/process/enterprise can tolerate an outage of a wide magnitude. Only someone familiar with your facility/process/enterprise can make that call. Switchgear is a lot more expensive, but that has to be weighed against downtime. Panelboards might be fine though if you have a facility/process/enterprise that doesn't have a heavy cost for being down.

Generally if you are a junior engineer, you would not be tasked with that kind of decision.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[davidbeach]

Yep, that's about right.

 

[waross]

It may be well to mention Available Short Circuit Current.
Your transformer should have an impedance voltage figure on the nameplate. It may shown as %imp, or as %Z.
This is the voltage required to force full rated current through the transformer with the secondary terminals shorted together. The %Z may range from about 1.8% Z to about 5 0r 6% Z.
Divide rated current by the %Z and this will be the Available Short Circuit Current.
EG: 75 KVA @ 240 Volts, 2.2 %Z. FLA = 75000 / 240 = 14,205 Amps Available Short Circuit Current.
All of your switchgear and or panels must be rated for the ASSC.
A helpful hint is to consider the impedance of the feeder cables. This will drop the ASSC and may allow the use of lower rated and cheaper equipment. When you are past the main panel or switchgear, you can consider the reduction in the ASSC by the impedance of the sub-feeders. You may be able to drop to a lower rating again if the cable impedance reduces the ASSC enough.
Another hint: cable is often cheaper than the next higher ASSC rated equipment. Don't be afraid to use cable length to reduce ASSC.
One installation was calculated for a minimum of 100 feet of cable between the main switchgear and the unit transformers. For the close in unit transformers we ran the cable past the unit transformer and then doubled back to use up the minimum 100 feet.

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

 

[rockman7892]

Jraef

I interpreted your "fault tolerance" consideration comment as a system coordination issue and the fact that Power Breakers used in Switchgear can have the Instantaneous setting turned off and therefore allow the short time delay to wait for 30cycles before tripping. With this you can set the main and even sometimes feeder breakers in switchgear with out an Instantaneous setting in order to allow them to delay tripping for downstream faults and keep the un-faulted parts of the system operational. I believe UL 1558 is the standard for Switchgear that allows these breakers to inhibit these delay characteristics.

In a panelboard or switchboard the molded case breakers have Instantaneous overrides above a certain fault threshold so its possible that the main breaker in a panel could trip for downstream faults above the breakers instantaneous region. This would mean that the whole facility would trip offline for a downstream outage instead of just the effected feeder breaker or downstream breaker.

This is how I interpreted you comments related to system fault tolerance and the probability of the facility staying online for downstream faults.

 

[jraef]

That's the issue, that power breakers in switchgear are expected to hold in longer to give downstream devices a chance to clear. Otherwise, the main breaker on a panelboard may trip and shut down the entire facility. That may or may not be a problem, that's what has to be decided on in chiding to use switchgear or not.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[GroovyGuy]

No easy answer here. Factors will include;
[ul]
[li]Application: ie utility, industrial or commercial[/li]
[li] Economics[/li]
[li] Load /Demand[/li]
[li]Client preferences[/li]
[li]Cost of downtime[/li]
[li]Arc-Flash considerations[/li]
[/ul]
In my industry, mid to upstream natural gas, we tend to not install LV Swgr. Instead we limit the transformer size to a max of 1500KVA and connect directly to the MCC bus, usually thru a MCC mounted main CB. This method provides a cost-effective solution and maintains acceptable AF levels.
In a previous life, we tended to max out with 3000KVA transformers, and provide LV metal-enclosed swgr for distribution to LV MCCs. That was many years ago, and I'm sure that he AF levels would not be acceptable today.

 

[yamyam]

Thank you guys so much for the response, I hope to apply all this knowledge that you have given me. Thank you guys. Oh and can you guys give me a sample of a design for a switchgear, I mean like a single line diagram of it? Thank you

 

[wroggent]

I know for one manufacturer that 4000A and 100kASCCR is the dividing line for switchboards and switchgears: if the main bus is rated 4000A or higher and more than 100kA SCCR is needed, then it must be switchgear. I would speculate that most manufactures have a dividing line in that neighborhood. To echo GroovyGuy, when I worked in the refining and chemical industries, common practice was to limit transformer size. Now working in the utility industry particularly on low voltage spot networks, I see many customers installing switchgear with high main bus rating and 200kA SCCR.