480V Current Limiting Reactors

[PWR]

A customer has several 800A rated or larger Motor Control Centers with 22,000A bus bracing and 39,000A available short circuit current. Has anyone had success using reactors at this voltage level to get the fault current down to an acceptable value. If so, can you recommend a manufacturer? Any other suggestions?

 

[dpc]

480V reactors ahead of MCCs were quite common back when molded-case breakers topped out at 22,000A. I've seen a lot of them. I'm sure Ritz and other reactor manufacturers could supply one.

But you're talking about a fair amount of reactance to get that level of reduction.

It's not the best solution, but sometimes it's the most practical.

You could also look at adding an upstream breaker that had a series rating with downstream breakers.

 

[gordonl]

When searching for the reactors look for drive isolation reactors, they commonly come at these sizes and can serve the purpose. (Again I've also seen it used for MCB installations)

 

[tulum]

Why don't you add current limiting fuses?

 

[REDDOG]

Adding reactors increasing the X/R ratio of the circuit.
Check with the manufacture of the MCC, for the short circuit bracing test X/R ratio. Is this test ratio within the new X/R parameters with these reactors.

 

[jbartos]

Suggestion: The larger suppression of the fault level by line reactors will add voltage drops and increase the X/R ratio. Therefore, current-limiting fuses may be considered. To avoid a single phasing due to fuses, a fused disconnect switch that would open the circuit during single phasing would be helpful.

 

[PWR]

Current limiting fuses are not much help at these levels of fault current.

For an 800A MCC, the necessary reactor will have less than 5 volts drop at full load and energy losses of about 500W. A 1200A unit has even lower drop with about 1000W loss.

X/R ratios are not much of a concern for several reasons.

 

[alehman]

Current limiting reactors are still available from most manufacturers as an option. See page 12...

http://www.ch.cutler-hammer.com/unsecure/cms1/TB04300003E.PDF

 

[RonShap]

A current limiting fuse upstream will not help, unless it has a series rating associated with it and the downstream breakers.

 

[jbartos]

Comment on PWR (Electrical) Mar 19, 2004 marked ///\\Current limiting fuses are not much help at these levels of fault current.
///Have you checked with

http://www.bussmann.com

http://www.ferrazshawmut.com

etc.? The Utilities use current limiting fuses to reduce the danger of large short circuit currents by limiting the short circuit currents to very low short circuit current levels all the time. I do too.\\\

 

[alehman]

In UL regions 800A could be a class T or class L fuse. Class T fuses provide significant current limiting. Class L provides much less. At 480V, 800A is the maximum size for class T.

As RonSharp said, in UL regions the fuse must have been type tested in combination with that particular circuit breaker. Circuit breaker manufacturers publish lists of tested combinations.

http://geindustrial.com/industrialsystems/wizards/seriesratings/home.htm

 

[PWR]

A Class K1 fuse rated 800A allows a peak instantaneous current "let through" of approximately 40,000 amps when the prospective short circuit current is 40,000A. Even still, because of the clearing time, you can use a 22,000A rated MCC according to some application literature with such a fuse. However, the latter will not allow any larger fuse to be used thus forcing another alternative at 1200A (some of which are present in the plant in question).

 

[CJFlatters]

Fitting upstream reactors or fuses may be considered, albeit they both have there problems and merits.

I am assuming that you have done a comprehensive system fault study to determine Ik" & Sk" at your MCC's to determine where your fault current contributions are coming from (SKM Power*Tools or similar).

If the high fault current rating Ik" is largely due to contributions from rotating electromagnetic machines (motor), the contribusion from these sources is approx S/X"d in kVA (ie LRC) for a cycle or so (where S = motor rated kVA).

Note - typically X"d = 0.167 for smaller induction machines (X"d FLC/LRC).

However, if you are using a lot of VSD's (not soft-starts with contactor bypass) the fault current contribution will be significantly lower due to SCR commutation failure when the bus voltage level collapses (speak to your VSD manufacturer - I am not a power electronics man).

Hope this helps



_______________________________________
Colin J Flatters BSc(Hons) IEng MIEE MIIE
Electrical Engineer / Project Manager
Email - cflatters@colin7.demon.co.uk.