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UL grounding requirements for equipment powered by daisy-chained connections?
martincho:
I am working on a device that will have AC power in and out connectors. The intent is to facilitate wiring multiple units to the same power circuit without having to use a multi-outlet strip.
This box needs to pass UL safety testing. I have been researching various sources and looking at several approaches to internal power distribution and, in particular, grounding or PE (Potential Earth).
There are myriad possible configurations. I have documented three of them below.
One of the criteria (both common sense and, I believe, as a safety requirement) is that a failure of PE should not propagate through a circuit. This leads to a star configuration, where a separate wire is used to go to a PE bar with multiple grounding screws.
However, this is a typical scenario for something like the internal wiring of an automation box with DIN rails, etc. This is different, this is a stand-alone device, the size of a shoebox, with power-in and power-out. Internally it has a power supply and embedded processor PCB and a few other items.
Imagine you have 5 or 10 of these linked and plugged into a single circuit breaker for AC. I am trying to understand how UL testing will consider various circuit configurations or if they have a preferred topology.
Here are three of the many options:
Single grounding lug; Splices for L,N and PE
The first example uses splices to connect power-in to power-out as well as the internal power supply. The PE splice contains an extra tap that then goes to the chassis ground stud via a single wire and lug, a star washer and lock nut secures it in place.
Here's an example of a splicing connector with push-in style ports. It is UL rated for 20 A, which means it is good for daisy-chaining a few of these boxes. The spring contacts are very reliable. Electricians use them. There are videos on Youtube of people torture testing these things to ridiculous extents.
Multiple grounding lugs on a single ground stud; Splices for L and N
This configuration wires PE from each connector and the power supply to a common chassis grounding stud. All connections terminate in a crimped circular lug. The hardware stack would include one or more external star washers as well as a nylon-insert locking nut appropriately torqued.
Uninterrupted conductors from AC-IN to AC-OUT for L,N,PE; Insulation displacement taps to feed power supply as well as chassis ground
In this case the connection from in to out is solid wire from connector to connector. A failure of a pin, crimp or conductor would have to happen for downstream devices not to have a connection to PE. Insulation displacement connectors are used inside the device to tap-off of the loop conductors. There are many excellent choices for IDC connectors that can carry the required current. Here's one of them:
The question is about what UL wants. In other words, this isn't about what is "best", which can be very subjective, even for something like this.
Yes, I know the first device will see the cumulative current draw from all series-connected devices. They only draw about half an amp, so it is easy to choose connectors and wiring to support half a dozen or more of these in series.
Thanks!
Gyro:
Definitely the second option (three tags to chassis stud). Incoming ground should go directly to the chassis.
jonpaul:
Most regulations for Earthing safety required a 25 amp AC current through the ground connection in case of a fault.
that current sould trip the mains breaker in cqse of short in device line or neutral to,case.
Suggest you posted your problem on the excellent Mile Holt Electric Inspection and code forum.
There you have professional Electricians and regulatory compliance experts.
Jon
martincho:
--- Quote from: jonpaul on February 27, 2022, 11:24:35 am ---Most regulations for Earthing safety required a 25 amp AC current through the ground connection in case of a fault.
that current sould trip the mains breaker in cqse of short in device line or neutral to,case.
Suggest you posted your problem on the excellent Mile Holt Electric Inspection and code forum.
There you have professional Electricians and regulatory compliance experts.
Jon
--- End quote ---
Thanks for pointing me in that direction. I will.
jonpaul:
ROD-L and other compliance instr makers have ground testers that pass 25A and check resistance of the connection.
Mike Holt forum https://forums.mikeholt.com/
Electricians forum
https://www.electriciantalk.com/forums/residential-electrical-forum.29/
https://www.electriciansforums.net/
Jon
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