LVSG (Low Voltage Swichgear)
LVSG (Low Voltage Swichgear)
(OP)
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!
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!






RE: LVSG (Low Voltage Swichgear)
RE: LVSG (Low Voltage Swichgear)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: LVSG (Low Voltage Swichgear)
RE: LVSG (Low Voltage Swichgear)
RE: LVSG (Low Voltage Swichgear)
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
RE: LVSG (Low Voltage Swichgear)
@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.
RE: LVSG (Low Voltage Swichgear)
RE: LVSG (Low Voltage Swichgear)
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
RE: LVSG (Low Voltage Swichgear)
RE: LVSG (Low Voltage Swichgear)
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
RE: LVSG (Low Voltage Swichgear)
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.
RE: LVSG (Low Voltage Swichgear)
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: LVSG (Low Voltage Swichgear)
- Application: ie utility, industrial or commercial
- Economics
- Load /Demand
- Client preferences
- Cost of downtime
- Arc-Flash considerations
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.
RE: LVSG (Low Voltage Swichgear)
RE: LVSG (Low Voltage Swichgear)