Supply Bus Isolation Advice

[leachdor]

Hi

I've got a situation where I will be drawing 255A from a 3-phase power bus. I'd like to use good design practice in its implementation, and I'm still in the learning process for power electronics. (computer engr. here)

I'd like a level of protection between the bus and the load. Correct me if I'm wrong, but if I was to rely on the bus plug fuses for protection, it would protect the load...however upon tripping...it would send a ripple down the line that could affect or damage equipment also on the bus.

I was thinking a 480 to 480 transformer. That way the line may be slightly more conditioned, and if it blows, it won't send quite the ripple down as if the entire bus plug went.

What do you think? Is it necessary for a transformer, or maybe a fusible disconnect would add that layer of protection? Any ideas that are in the same price area or cheaper?

Thanks much!

 

[davidbeach]

What "ripple" are you concerned with? What damage are you concerned with preventing?

 

[leachdor]

Ripple was made reference to by another engineer here. He will be out of the office till next week, but its something that needs to be figured out by Friday. He seemed quite confident, one of those responses that he just assumes everyone knows...lol. Apparently when the fuses blow on our bus line that are carrying 600-2000A through them, and passing 250A through the fuses inside...I guess at that moment a spike back through the bus is a result. Couldn't tell you the theory behind it, but its general practice all over the plant to isolate high current drawing components with more than fuses. :-/

Damage to be prevented is under the assumption this large spike does exist. Further down the bus it could affect the highly sensitive data acquisition equipment we use, or trip an old fuse on a sensitive high priority production machine, mess up a calibration on something dedicated to precision...I guess it could do all sorts of things...I think its less worry about fireworks and dramatic things as it is messing up something small that would affect product.

Its all under assumption that he was right, he's been at it 30 years though...I'm still pretty green and this isn't what I was hired for lol.

 

[dpc]

At 480 V, I don't see any problem using a fuse or molded-case circuit breaker for circuit protection. It's done all the time.

But your resident guru should know your system better than we do.

If you are also concerned about power quality issues from whatever it is this feeder is serving, the fuse or breaker will do nothing about this.

 

[jghrist]

If there is a problem with voltage spikes, the way to protect equipment is with Transient Voltage Surge Suppressors (TVSS). The most likely voltage transient during a fault on a feeder is a voltage depression, caused by the fault current flowing through the system impedance.

 

[sslobodan]

Fuse is Ok for your application, especially as I believe your equipment is connected to an UPS to prevent data loss. If it is a large data center you might even have AC/DC/AC direct conversion (in best possible case) that is creating his own power grid within very nice parameters (pure sine wave and clear voltage on 230V phase to neutral was in 2 places where I have seen it). You don't have to worry, problem starts with motors connected to the other end and the biggest motor you have is probably small chiller(s) to cool down your gear. fast circuit breaker on 1.2 of nominal current for computer gear is OK. and ups is MUST because of data loss, and he will rectify all "Ripples" that network may create because even the cheapest Chinese UPS units has that option now (they actually give you 230V rectified with capacitors and battery power reserve instead of line voltage)

P.S. it works both for EU(230) and US (120V) market I just wrote EU values because I am from Europe. Principles are the same