Electrical enclosure bonding.

[RoarkS]

So... I'm working on a box that encloses a DC to AC 3 phase motor controller. The controller card itself is mounted to an aluminum finned heat sink. I'm not 100% if the stainless mounting screws make the heat sink "electrically bonded" to the card... I'm guessing not as they are stainless and I want to say in an email I have from the EE it isn't... so lets assume it's not. It's also type 2 clear anodized.

The finned heat sink forms the lid of the box. The actual box itself is just a box with a raychem cable entry boot and a mil flanged circular connector. The box is also aluminum and type 2 clear anodized.

The idea is there is a RTV gasket between the two and they are riveted together with stainless pop rivets (cherry brand but not cherrymax).

Is there any reason to bond (.0025 ohm) the two and ground the assembly to the airframe/ground?

I know it's an EE question but it's weird aircraft stuff.

 

[Comcokid]

RoarKs-
Since no one else is responding, I will.

DO-160 requires for conducted/radiated emissions testing that a resistance measurement be made from the UUT to the test table to ensure it is bonded to the table. To do this they will sometimes scrape the anodization from a spot on the enclosure and measure from the enclosure to the table. This is a test-setup requirement, not a pass/fail requirement. If there's trouble at one spot, they may try another. There have been times on products I have designed, for one reason or another, they have trouble getting it under the milliohm limit, so it is just noted in the test report. Even with a good calibrated milliohm meter there are times you just can't seem to get good contact.

As for the bonding between parts of your product, as far as I know that is your business. If there is high current known to be flowing between the parts, then a milliohm bonding design requirement may be needed, but that's again your own internal business. The only time I've encountered trouble there is on air-ambulance aircraft a CE spec for hospital-use medical equipment will be applied to every external metal surface, but that's more of a negotiation item in a contract since they are applying a spec outside of it's normal area.

 

[KSshopper]

I don't know anything about the specifics of your box, but the case needs to be bonded to the aircraft ground plane. See SAE ARP 1870 for more guidance.

 

[WKTaylor]

In addition to DO-160 and SAE ARP1870... spaghetti on the wall..

Enclosures...
ARP1481 CORROSION CONTROL AND ELECTRICAL CONDUCTIVITY IN ENCLOSURE DESIGN
MIL-E-24142* ENCLOSURES FOR ELECTRICAL FITTINGS AND FIXTURES,... ...
MIL-E-24743 ENCLOSURE, COMMON ELECTRONIC EQUIPMENT, GENERAL SPECIFICATION FOR
MIL-STD-108 DEFINITIONS OF AND BASIC REQUIREMENTS FOR ENCLOSURES FOR ELECTRIC AND ELECTRONIC EQUIPMENT
IEEE STD 299 * METHOD FOR MEASURING THE EFFECTIVENESS OF ELECTRO-MAGNETIC SHIELDING ENCLOSURES
IPC-A-630 ACCEPTABILITY STANDARD FOR MANUFACTURE, INSPECTION AND TESTING OF ELECTRONIC ENCLOSURES
IPC-HDBK-630 GUIDELINES FOR DESIGN, MANUFACTURE, INSPECTION AND TESTING OF ELECTRONIC ENCLOSURES
NFPA 496 STANDARD FOR PURGED AND PRESSURIZED ENCLOSURES FOR ELECTRICAL EQUIPMENT
Electrical Bonding & Grounding
AIRCRAFT WIRING AND ELECTRICAL BONDING, ISBN 9780983865803
AC25.899-1 ELECTRICAL BONDING AND PROTECTION AGAINST STATIC ELECTRICITY
FAA FO 6950.19 PRACTICES AND PROCEDURES FOR LIGHTNING PROTECTION, GROUNDING, BONDING AND SHIELDING IMPLEMENTATION
FAA FO 6950.20 FUNDAMENTAL CONSIDERATIONS OF LIGHTNING PROTECTION, GROUNDING BONDING AND SHIELDING
FAA-STD-020 TRANSIENT PROTECTION, GROUNDING, BONDING AND SHIELDING REQUIREMENTS FOR ELECTRONIC EQUIPMENT
MIL-HDBK-274 ELECTRICAL GROUNDING FOR AIRCRAFT SAFETY
MIL-HDBK-1857 HANDBOOK FOR GROUNDING, BONDING AND SHIELDING DESIGN PRACTICES
MIL-STD-464 ELECTROMAGNETIC ENVIRONMENTAL EFFECTS REQUIREMENTS FOR SYSTEMS
MSFC-HDBK-3697 ELECTRICAL BONDING DESIGN GUIDE HANDBOOK
MSFC-SPEC-3659 PROCESS SPECIFICATION FOR ELECTRICAL BONDING
AG CASA AC 21-99 AIRCRAFT WIRING AND BONDING
NATO STANAG 3659 ELECTRICAL BONDING REQUIREMENTS FOR METALLIC AIRCRAFT SYSTEMS
A-A-59398 SHIELDING GASKET, ELECTRONIC [POLYMER GEL + WOVEN METAL WIRES]
MIL-DTL-83528/* GASKETING MATERIAL, CONDUCTIVE, SHIELDING GASKET, ELECTRONIC, ELASTOMER, EMI/RFI


Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[RoarkS]

That's a lot of spaghetti sir. Much appreciated, that'll take me a minute.

 

[IRstuff]

Is there any reason to bond (.0025 ohm) the two and ground the assembly to the airframe/ground?

The simplest answer is "YES." There are simple steps you can take to minimize EMI/EMC susceptibility/emission

> mask the mating surfaces from anodize or grind off
> use irridite or conductive chem film
> ground both parts of box to cable shield

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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