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Where I work we manufacture a machine delivered as a stand-alone enclosure comprising inside a small control panel which holds some relays, the PLC, the HMI, and terminals for controls. The rest of the power components are laid out inside the main stand-alone enclosure (MCCB, VFD, Input reactor, sine filter, transformer, etc.). The customer needs to bring 480V to the machine with the option to bring the cables in through the top, directly to the MCCB, or through the bottom to a set of terminal blocks factory connected to the MCCB. My question is regarding the UL508A requirement for the SCCR. In my mind, coming through the top, we achieve 65kA due to our MCCB. Other components downstream are sized to handle it too, in the way they are protected. No problem.

My concern is when power is fed from the bottom and brought to the terminals first. They are Weidmuller WFF300, and their SCCR is that of untested power distribution blocks, which is 10 kA according to table SB4.1 of UL508A. This is clearly a bottleneck for me, unless we can disregard these, as they are themselves enclosed in a sub compartment of the main stand-alone enclosure. I read again and again on the matter and could not find a satisfactory answer to this specific case. Obviously, in some installations, it will make a huge difference if the customer specs out a SCCR the unit should be capable of handling.

Anybody knowledgeable with UL 508A SCCR?


Since I am not that knowledgeable with UL 508A panel construction, but I do not see why terminal blocks would need a SCCR rating.
The only time you worry about SCCR is when the device is breaking current flow. Terminal blocks are not designed for SCCR. They designed for rated current flow, voltage, wire or lug size, ect.
UL has them listed for 500 Amps and 1000 Volts

IMHO I see no problem with using the stated TB for 480 volts and 500 amps.
Hope this helps,


UL508A requires that the nameplate of a machine displays the Short Circuit Current Rating of its industrial control panel (Art. 670.3). The means of determining this SCCR for me is through an approved method, but I butt against the terminal block’s low SCCR then, or so I think. That’s what I’m unsure of, since they are part of the machinery at the end.


I did some research on the topic at hand and you are correct about SCCR for terminal blocks.

Check Allen-Bradly 1492-PD Power Blocks. Also Automation Direct for TB's, which has two or more brands with higher ratings.
Google "terminal blocks"
Hope this helps,


Yep without a current limiting fuse or circuit breaker the 10kA default rating will pull down the overall SCCR. As melspuds mentions, there are higher SCCR terminal and distribution blocks available.

And yes terminal blocks are part of the SCCR equation.


Based on UL 508A, any component in the power circuit (i.e. a component that is in a circuit path leading to an external load) should be considered when determining the SCCR of the control panel. It is not just the circuit breaker that must be considered. It is all power circuit components. Terminal blocks carry a 10kA default value unless the terminal block used has an SCCR, and is protected by the overcurrent protective device as specified by the component SCCR. It is possible to raise the SCCR of components in the branch circuit using an upstream current limiting fuse/circuit breaker, or using a transformer. However, there are restrictions that must be followed. If you like, email FUSETECH@eaton.com for additional support, or reply to this post. #SCCR #UL508A


Quote (OP)

The only time you worry about SCCR is when the device is breaking current flow. Terminal blocks are not designed for SCCR.
More about the history than the actual ratings:
Years ago when I was starting, the old text books and old timers had stories of incidents where the magnetic forces of short circuit current tore bus bars loose or caused conductors to contact phase to phase or pull free of the fastening.
On bus bars; Klockner Mueller had a pamphlet with a series of stop action photographs of the movement of bus bars due to the magnetic forces of a fault current. The center bus bar broke free and contacted the upper bar. Then it was repelled and contacted the lower bus bar.
On conductors; Years ago an old timer told me a story of an incident when he was working in a substation in a Major city years before.
"It was a dark and stormy night." With high winds and ice buildup on the transmission lines. Several transmission lines were out and those still in service were badly overloaded. One of the lines in service was a 60kV pole line from a small hydro plant 40 or 50 miles out of the city. The wind apparently caused two conductors to contact phase to phase. Two things happened,- The arc burned the conductors free and the current caused the other span conductors to repel each other. When the arc cleared, the next span swung together and the process was repeated. I assume that the greater current was the backfeed from the grid. When the smoke cleared, several miles of line had each span either down or damaged.
A lot of effects that are now taught and protected against were originally discovered by accident years ago.
One of the more recent issues would be the massive domino effect power outages of major grids within the last 50 years or so.
Schemes such as Jungle Mux and similar are used (in some cases with fibre optic communication links) to anticipate and avoid future domino effect outages.
Yes! SCCR is inportant for terminals and components.

"Why not the best?"
Jimmy Carter

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