Panel & Enclosure Grounding in a Large Instrument

[leonhart88]

Hi,

I have a general "best practice" question about grounding of large instrumentation. In the past, when I've built enclosures or sub-systems out of 8020 and aluminum or powder coated steel panels, I've always attempted to ground all panels & metal parts (eg. using PEMs on the steel panels, and biting washers on the 8020 extrusions with grounding wires).

You can imagine that for a large instrument, this can start to become a ton of work with lots of little tooth washers and nuts, etc. One can also imagine that various parts of the instrument will never have high voltage components inside of it and that the high voltage components can be isolated into one section of the instrument.

In said system, there's one inlet connector for 120V AC going into a AC/DC power supply and then low voltage DC (24-48V) is broken out to various sensors & actuators inside.

My question is, is it best practice to ground every single metal item in this instrument? Or is it adequate to simply limit the area in which the live AC wires could touch during a fault and only ground that subsection? For example, if the AC wiring is confined to a small sub-section of the 8020, is it adequate to just ground the extrusion & panels around that area? Alternatively, if the AC wiring enters an enclosed box (like a junction box) immediately and the AC/DC converter is located inside of said box, with only 24-48V DC exiting the box, do any external panels need to be grounded at all?

Note that I'm just curious about this as a best practice, however if there are applicable standards for general instrumentation, I would be interested to know details on those as well.

Thanks everyone!

 

[drawoh]

leonhart88,

What are you trying to accomplish? If you are concerned with EMI/RFI shielding, you need a grounded enclosure.

--
JHG

 

[leonhart88]

Hi drawoh,

Sorry I forgot to mention what I was trying to accomplish...this is for safety, not EMI/RFI shielding. The most applicable standard would be something like IEC 601010.

 

[IRstuff]

Personally, I like everything grounded; if for nothing else, so that I don't get the buzzy feel of AC-coupling to the panels. From a pure safety perspective, grounding is also desirable, in case insulation wears out or there are dangling/loose wires or filaments that can cause an inadvertent connection to the chassis. Obviously, if one were adhering to strict single-point-ground and EMI best practices, the chassis would be grounded. 48V is supposedly above the military 30V spark hazard limit, so that's something to consider.

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[itsmoked]

Gack. A panel made from 8020..

Every part of the enclosure that's metal should be grounded.
Are the walls metal?

This is a common problem with 8020 and since they have billion specific bits and pieces to solve every crazy need I'm surprised there isn't one specifically for grounding. Have you checked?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[drawoh]

leonhart88,

Safety from what? This sounds like you need to meet NEMA[ ]4 or one of the IP[ ]ratings.

--
JHG

 

[leonhart88]

So, if I refer just to something like IEC 61010 (safety requirements for electrical equipment for measurement, control and laboratory use), I'm talking about protection against electric shock (section 6 in the standard).

The standard basically says 2 things:

- You must not allow any ACCESSIBLE parts to be HAZARDOUS
- You must provide protection in case of SINGLE FAULT CONDITIONS

The standard says anything above 60V DC or 30V AC is categorized as HAZARDOUS in dry environments if certain current or capacitive limits are exceeded. So for low current voltages below 60V it isn't classified as HAZARDOUS.

My understanding of this means that if you have 24V wires floating around, it doesn't matter whether it's accessible or whether it touches conductive panels under a single fault condition because they aren't classified as HAZARDOUS according to the standard.

The only thing you need to do is provide protection around the HAZARDOUS areas under single fault and make sure they aren't freely accessible. The standard says you can do this by PROTECTIVE BONDING (eg. earth ground bonding), INSULATION, ENCLOSURE, etc. Hence my comment earlier about limiting the area in which AC HAZARDOUS components & wires are located (eg. putting a grounded cover over this area, or an enclosure like an NEMA rated junction box).

I'm fairly certain I understand the requirements of the standards after reading it several times. This also kind of makes sense because if you have a huge instrument with lots of metal panels, etc. it does seem a little arduous to have to ground every single thing with a conductive ring terminal. I still wanted to ask and see if someone who had a lot of experience in this area could confirm though. Thanks for everyone's input!

Also, you can buy nuts and accessories for 8020 which bite into the anodizing specifically for grounding purposes. 8020 sells them and MISUMI also sells their own version of these.