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]

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

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

[EPAIII]

I am not by any means an expert on the US NEC or getting UL or other approvals for new designs. But I have worked on thousands of devices that use AC power, some of which had utility or pass through outlets for the same. I have also built custom equipment which was AC powered, mostly on a one of a kind basis or a small quantity for my company's internal use.

I have never, in over 45 years of experience, seen any type of electrician's splicing methods inside any of them.  If there is a typical way in which this is handled it is by placing the IN and OUT connectors more or less side by side and using SOLDERED wires between them. Some such connectors will have terminal arrangements that are designed for this type of busing. Of course, there was also a wire going directly from the AC IN connector to chassis ground if the connector itself did not have that as an internal connection. This was often just a continuation of the wire that bused the safety ground from one connector to the next.

In some cases this safety ground connection at the chassis was made with a crimped terminal and a screw. Often that screw was also one of the ones retaining the AC connector to the chassis. Crimped terminals are also used in some instances for the AC connections to a transformer, fuse, or circuit board. But I can not recall seeing them used to connect the wires to the AC IN or OUT connectors: those were virtually always soldered. This is not to say that such things do not exist, only that I can not recall seeing them.

A older, further remembrance is of the AC cords passing through the chassis with a strain-relieving grommet and terminating at a nearby terminal strip which was firmly attached to the chassis. In some such cases, where no AC connector was employed, the terminal strip was a solder terminal type and one or more of the terminals also connected to a mounting lug(s). These were used for the required chassis connection. A main point here is that the terminal strip was mechanically attached firmly to the chassis and not allowed to dangle in mid air as those electrician style termination devices almost always do. This makes me seriously wonder if such unattached connection methods are even allowed inside a chassis.

As for daisy-chains, everything I have seen has avoided this to the greatest extent possible.

All of this, while the product of many years of working with electronic devices, is not necessarily indicative of code or other requirements. You should obtain and read the appropriate documents. In the US, the latest copy of the NEC would be one, but it will not provide all the details. I am sure there are others that would be even more on point.

AND, what UL wants can be best addressed by asking UL or reading their published documentation.

[martincho]

I have never, in over 45 years of experience, seen any type of electrician's splicing methods inside any of them.

The methods I propose are not what I would consider exclusive to electricians wiring a building.  There are entire families of UL-rated splicing and tap connectors designed for this kind of duty.  The question is more about what UL thinks is the safe or safest way to implement something like this.  Ultimately, yes, the testing lab will have to have input on this.  I am about to start working with them.  I thought I'd ask around a bit and see if I run into useful insight.

The various UL standards I have reviewed include specifications for the type of wire one can use, size and such things as distances to both conductive and non-conductive elements of the product interior.  I feel these areas are very well defined and relatively easy to research through the standards.  I have not yet identified a standard or portion of one that deals with daisy-chaining power. 

An outlet strip or multi-tap extension cord might be the closest reference.  These are covered by UL 1363.  One interesting data point from section 21, Grounding, is that "A push-in (screwless), quick-connect, or similar friction-fit connector shall not be used for this connection".  In the same section they list "clamp, bolt, screw braze weld or equivalent positive means that cannot be loosened from the outside" as approved connection mechanisms. Soldering is only allowed if the supply cord ground wire is mechanically secured to the enclosure.  This makes sense because soldered connections don't do well with any kind of flexing (the conductors crack, break).  Other interesting notes from UL 1363 include that any soldered connection must be mechanically secure before soldering (you can't use solder mechanically).

While UL 1363 seems relevant and is interesting, it applies to "Relocatable Power Taps", aka.: power strips.  That said, there probably is a solid argument that a daisy-chain power configuration turns the piece of equipment into a relocatable powr tap.