S.C.C.R. of control panel for machine control ?

[milmat1]

As an OEM of industrial machinery we build our own panels and control systems in house. I am trying to get this shop up to speed on proper nameplate
installations on our cabinets. Which up until now has not been done at all..

Concerning the SCCR rating, I have been studying how to calculate these ratings and I have created a great deal of confusion...

Using the Idea of "weakest Link" , the lowest rated component in my cabinet can in many cases determine the SCCR. But that component (lets say its a overload or a motor starter or Dist block etc) is actually
behind several other protective components. Such as the main disconnect fuses / main breaker, MPCB etc or maybe a transformer. In other words it would never see the AFC from
the supplied power source. So why do the components downstream of the main OCPD control the SCCR rating?

I'm not wording this well I understand, But I'm trying to get my head around this SCCR rating and it's purpose. For example if a set of motor leads coming from the cabinet were to become faulted together or to ground etc. That fault current would be limited by several devices long before the main OCPD even seen a problem? So the supplied power and it's AFC would never come into play would it ?

Can anyone give me a kind of overview of the SCCR nameplate rating and how that rating interacts with the field installation of the machine / control panel...

 

[petersonra]

The bottom line is this.

The SCCR of an industrial control panel (ICP) is determined by a procedure set up by a NRTL. At present, the only one I am aware of is UL508a. So, if you want to determine the SCCR of an ICP you MUST use the procedure found in UL508a to do so. You have no other option unless you want to make a sample of every ICP you make and submit it for testing to a NRTL. This is neither practical nor economical, so you are left with UL508a.

It is not the responsibility of the OEM to determine what level of SCCR is required for a specific ICP, only to determine by an appropriate procedure (in this case UL508a) what the SCCR of the ICP is.

The owner is responsible for making sure that the short circuit current at the line terminals of the ICP is less than or equal to the SCCR of the ICP. The calculation of what the short circuit current at these line terminals will be includes everything up to that point including length and size of conductors, transformers, etc.

I would not make a big deal of this. These days there are series combinations of products that are readily available for 65kA. You can probably make your ICP have a SCCR of 65kA with little or no increase in cost just by getting rid of junk you really don't need anyway, like distribution blocks and terminals in the power circuits, assuming you are not using junk for motor starters.

I will tell you this. The procedure for determining the SCCR of an ICP found in UL508a requires that the ICP meet all the other requirements found in UL508a. While these requirements are not onerous, or costly, there are a lot of OEMs who do not follow them.

 

[petersonra]

Using the Idea of "weakest Link" , the lowest rated component in my cabinet can in many cases determine the SCCR. But that component (lets say its a overload or a motor starter or Dist block etc) is actually
behind several other protective components. Such as the main disconnect fuses / main breaker, MPCB etc or maybe a transformer. In other words it would never see the AFC from
the supplied power source. So why do the components downstream of the main OCPD control the SCCR rating?

The weakest link is the weakest link.

However, many "weak links" are able to have a series rating that far exceeds their own rating if there is a specific fuse upstream of it. You might have a bunch of motor starters and overloads that are only 5kA SCCR on their own but are 65kA with a class J fuse upstream. usually the UL listing will give a maximum value of the fuse that you can use to get this rating Suppose it says 60A class is the maximum. You do not have to have a 60A class J fuse on every motor starter. just have to have one somewhere upstream. You could have 100 motor starters fed by that one fuse and it would still count for the higher series rating.

 

[Jraef]

The weakest link is the weakest link.

However, many "weak links" are able to have a series rating that far exceeds their own rating if there is a specific fuse upstream of it. You might have a bunch of motor starters and overloads that are only 5kA SCCR on their own but are 65kA with a class J fuse upstream. usually the UL listing will give a maximum value of the fuse that you can use to get this rating Suppose it says 60A class is the maximum. You do not have to have a 60A class J fuse on every motor starter. just have to have one somewhere upstream. You could have 100 motor starters fed by that one fuse and it would still count for the higher series rating.

The same is true for many, if not most, other power components now. When article 409 was added to the NEC in 2005, a lot of suppliers were caught off guard and many devices were not tested, or at least series tested, leaving a lot of panel builders in a lurch or making them put on the "courtesy" 5kA SCCR label, which is all but impossible to attain in the field. So once mfrs started testing and getting higher ratings on their components, panel builders began using them, which meant the suppliers who didn't bother were losing out. Most large mfrs have now caught up, but some of the cheapest of the cheap bottom-feeders have not, because their market is people who don't care about anything but price. So if you come across a device with a low SCCR and/or no series listing, look around, you can probably find one that has it.

And...
If you try to mix components from different suppliers based solely on price, you will get yourself in trouble. Series ratings are attained by testing components together. So Allen Bradley will test their contactors with their overloads and their circuit breakers, so you can get a 65kA series rating on the combination, even though the OL alone is only 5kA and the contactor is 10kA. But A-B is not going to pay to test their OL with a Siemens breaker and a Telemecanique contactor.

 

[petersonra]

And...
If you try to mix components from different suppliers based solely on price, you will get yourself in trouble. Series ratings are attained by testing components together. So Allen Bradley will test their contactors with their overloads and their circuit breakers, so you can get a 65kA series rating on the combination, even though the OL alone is only 5kA and the contactor is 10kA. But A-B is not going to pay to test their OL with a Siemens breaker and a Telemecanique contactor.

They probably won't fit anyway.

What I have discovered is that most of the time the least costly option is a type F starter, if one is available for the size motor you are using. However, I would point out that even some name brands have some problems in the smallest and cheapest sizes. For instance, the smallest Siemens type F starters are not really type F because they are not UL listed that way (you have to look very carefully to realize this).

Another issue with type F starters is that they cannot be used on 480V delta systems. They are only listed for 480/277V systems.

I have seen OEMs that use regular relays to switch motors and put a supplemental CB on it, presumably as the overload.

 

[milmat1]

Thanks everyone for the replies and explanations, As I study this subject I am slowly learning more about these ratings.