Circuit Breakers, 480Y vs 480 Delta, What is the difference. 7

[elfman]

Let me preface with the fact that I have an MS in Mechanical (run away) but have seen the light and am converting to the controls world. Long story.

The control panels I design often control multiple motors (branch circuit) and as such I need to provide fuses/breakers on each branch. We normally are taking in 480VAC power and from the reading I have done, it seems like the standard in the US is Delta. However, I have not been able to find any miniture circuit breakers that will handle anything but 480Y. Why? What is the difference? And does anyone know of affordable, good circuit breakers that are compact and rated for 480 Delta? What are my options here, fuses, especially when specs often call for significant numbers of spares, are just not economical?

Thanks in advance!

 

[elfman]

Let me add, I know I can use the Molded Case Circuit breakers, but they are generally expensive, have a large footprint in the enclosure, and don't come in anything but large increments, meaning I have to way oversize, which makes me uncomfortable. I am looking for some instruction and opinion here.

 

[resqcapt19]

The difference is that the 480 wye breakers are rated 480/277 volts and can only be used on systems with a maximum voltage to ground of 277 volts. The delta breakers are for use on a grounded phase system or an ungrounded system and are rated for 480 volts to ground.

 

[davidbeach]

Even if only the three phase conductors are present, most North American 480V systems are wye, not delta. Look at the transformer that supplies the 480V, it will have a 480 wye winding far more often that it will have a 480 delta winding.

 

[ScottyUK]

Watch the fault breaking capability of the MCBs. Most European types are 10kA, apart from the domestic / light commercial types which are often 6kA. You don't need an especially large transformer and feed cable to exceed 10kA if your panel is electrically close to the source.


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[elfman]

All good, informative responses thus far, please keep them coming. More specifics and ideas on what products to use for the branch circuit protection would be great, too!

 

[jraef]

The difference is that the 480 wye breakers are rated 480/277 volts and can only be used on systems with a maximum voltage to ground of 277 volts. The delta breakers are for use on a grounded phase system or an ungrounded system and are rated for 480 volts to ground.

elfman,
What that means (in mechanical terms because you are not an EE) is that a "slash rated" breaker (480/277) is possibly incapable of physically holding together during a phase to ground fault where there is no ground reference. In a Y system such as 480/277Y, the maximum voltage from any phase to ground is always going to be 277V, because the SOURCE is already referenced to ground on all 3 phases. The phase to ground voltage in a Delta system however can be as high as the line voltage, i.e. 480V. The amount of physical force exerted on the components inside of the breaker is proportional to the ENERGY in the event, not just the current. So if the voltage is higher, the same amount of current will represent a lot more energy and therefore a lot more magnetic force exerting mechanical strain on the breaker components. That force can destroy a breaker, and UL rules require that they be capable of being reset after a fault they are rated to interrupt.

ALL of those MCBs (Mini Circuit Breakers) are made for and by the IEC market and although they have been sold here for years, they were originally only rated at 415/240V max. Recently (i.e. 5 years or so) some of the mfrs re-tested the breakers at 480/277 and got UL listing that way, but they CANNOT pass the UL test standards for a delta system; too much energy. Translate: light duty if you will. In places outside of North America, ungrounded Delta systems are extremely rare, so IEC component manufacturers generally don't need to worry about this. But when they started selling into the US and Canada, they wrongly assumed that all of our systems were 480/277Y systems as well, because that is the current standard for NEW systems. However there are a great many legacy systems still in use that are 480V delta, open delta or what's called corner grounded delta. In all of those systems, you CANNOT use slash rated breakers. There is no MCB I am aware of that is not slash rated. If you look at the physical construction differences between the MCBs and the MCCBs, you can see why, there isn't enough "there there". So unless you are absolutely sure of where your control panel is going, it's not a safe bet to use slash rated breakers. You will see a lot of high tech (i.e. semiconductor industry) control systems where they use these extensively. This is because a) most of the installations are being done outside of the US even if the systems are built here, and b) even if they are being used here, it's a relatively good bet they are being hooked up to a NEW service that will be 480/277. If you are not in that industry, it's risky.

Another aspect that you can no longer ignore if you are building motor control panels is the SCCR rating and labeling issue. Most MCBs are rated at only 10kAIC, some as high as 18kAIC, but in general that is often considered very low. Then if you look at most OL relays, another component in the motor power stream, you will see that many of them are only rated above 5kA when used with fuses or MCCBs, rarely with MCBs. If you are building motor control panels, you need to read UL508A Supplement SB, and/or the NEC article 409. The basic rule is that unless you want to go through a $20-25K short circuit withstand test with UL for every different panel you build, you are going to end up with an SCCR rating that is as low as the lowest rated component in the box. If that is the OL relay because you used an MCB, that means when your user installs the equipment the inspector will insist on them proving that there is no more than 5,000A of fault current available. That is an extrememly difficult number to attain. Most residential installations are 10kA as an example.

A few of the major mfrs do have low cost delta rated MCCBs for low AIC applications. I used to work for Siemens, they have a line called the NGG Series; DIN rail mountable, not much larger than an MCB and they are delta rated for 25kAIC if memory serves me. Many of their competitors have similar offerings, I'm just not versant in them.



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[elfman]

Awesome response jraef. Thanks. Clears up a lot!!

 

[jghrist]

jraef,
Would the same apply to a wye HRG 480 volt system?

 

[davidbeach]

Yes, the slash rating is only good for a solidly grounded system.

 

[jraef]

jraef,
Would the same apply to a wye HRG 480 volt system?

You know what, I don't know. After pondering that a bit, I came to the conclusion that it would apply (meaning you cannot use slash rated breakers on an HRG system). The purpose of an HRG system is to limit gound fault current, which would at first seem to be OK. But as I recall, during a phase to ground fault on one phase the voltage reference to ground on the other two phases goes to line voltage. So if a slash rated breaker tried to open, 2 of the 3 poles would be subject to that higher energy level. No good.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

LOL, So I pondered correctly then... Thanks David.

 

[elfman]

Pardon my ignorance, what is an HRG system?

 

[davidbeach]

High Resistance Ground(ed)