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Topic Title: Equipment earthing
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Created On: 07 November 2017 10:10 AM
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 07 November 2017 10:10 AM
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GaryND

Posts: 5
Joined: 06 November 2017

Hi all,

My first post so please be gentle.

I have a question about equipment earthing which hopefully one of you fine people can answer or point me in the right direction. I am working on the design of a 18" rack mounting chassis which uses plug-in server type power modules. These modules are fully contained with IEC mains connectors and all the relevent safety standards. The power module slides into the rear of the chassis where it plugs into the low voltage connector (12V) and has a copper spring strip compressed to the chassis. The chassis picks up its earth connection via this spring strip to the power module earth connection. So apart from the power module the chassis is all 12V or lower. My question concerns the front panel, this panel is a detachable plastic moulding with a large steel venterlation grille. The moulding has an EMC coating internally to which the grille connects and conductive fabric gaskets to the chassis when in place. Question, do I need an earth wire to this grille from the chassis as this would be inconvenient when removing the front panel.

Many thanks
Gary
 07 November 2017 10:42 AM
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AJJewsbury

Posts: 16014
Joined: 13 August 2003

The decision to earth something or not isn't always a simple - firstly you need to think about why you might want to earth something. The usual reason of course is to protect against electric shock, but you might still want to earth something that didn't need shock protection - other reasons might include reducing EMI, providing low impedance mesh for functional reasons, a duplicate earth path for other components where there are high earth leakage currents or occasionally even for an earthed return for ELV power. IT and telecoms equipment often has additional bonding conductors for one or more of the 'other' reasons.

For shock protection the basic criteria is whether the part would be liable to become live if basic insulation failed. Normally insulated & sheathed flexes would already provide adequate protection (provided sensible measures are taken to ensure a single event doesn't cut through both the sheath and insulation - so no sharp edges on a metal door for example). Metal cased mains powered equipment normally has its case earthed (or is double insulated internally) - so no further earthing is needed for other metal in contact with that to guard against shock from faults within the equipment.

The 12V system (which incidentally is classified as extra-low-voltage (ELV), Low Voltage (LV) is anything between 50V and 1000V a.c.) may or may not need consideration from a shock point of view depending on how well it is separated from the mains - normally is would be separated to a standard equivalent to double insulation and be classed as SELV (or occasionally PELV) - which makes the shock risk negligible, however there are ELV systems that could under fault conditions (e.g. a short between the LV and ELV sides of the PSU) put mains voltage on the 12V system - in which case some additional precautions (possibly including earthing of the 12V exposed-conductive-parts) would be needed.

- Andy.
 07 November 2017 11:02 AM
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GaryND

Posts: 5
Joined: 06 November 2017

Thanks Andi for the quick reply. The grille sits infront of the cooling fans and it conductive purely for EMI/EMC reasons. For this reason it connects to the chassis/Ground but only by way of the EMC coating and gaskets, a few 10s of ohms. No way would it carry 25A or what ever the require test current would be. You can probably tell I'm very hazy when it comes to standards. The grille is also anodised, although I don't think that would give must insulation. For the grille to beome a shock hazzard it would need the PSU barrier to fail as well as the earth connection, there are two PSUs so the ground connection would need to fail on both units simultaneously.

Gary
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