generator/large motor ground returns 1

[jstolp]

Hello,

I am looking for some guidance concerning airframe bonding-ground return for a large motor. I am working on a twin engine, turbo-prop that is undergoing an airframe overhaul. This aircraft has an aluminum fuselage with the engine mounts made out of welded steel tubing. All the structural components in the engine mount to airframe interface are of course heavily primed/painted. Each engine has a 28VDC starter/generator. Additionally, a main battery and various other electrical components are in each nacelle area. I need to determine what materials, (flexible braided straps, solid-flat plated copper straps ??) and what sizes are required. The generator output is rated @ 200 amps and I believe the starter pulls in the area of 400 to 600 amps (short duration). Also where do I need to stop with the bonding (large current ground return)? Does the wing (1 piece) to fuselage attach point require attention too?

Thanks for the help

J. Stolp

 

[BrianR]

In the absence of other requirements you will get some very useful information from MIL-W-5088 for wiring and MIL-B-5087 for bonding and grounding. They are available at

http://131.82.253.19/quicksearch/.

You should install high current bonding between each engine and wing, wing to fuselage and fuselage to generators/ battery/ inverters/ ground power connector etc.

Mil 5088 will give methods for calculating the bonding cable cross section and materials, but in general tinned copper is the material of choice.

Mil 5087 will show methods of attaching ground leads to structure. At the bonding point, clean back to bare metal then brush on a conductive alodyne coating to give some corrosion protection before making the connection. Some locations may require waterproofing too.

Do not rely on the engine mounts for the current return but install a dedicated ground lead across the mounts. Structural requirements call for good quality corrosion protection at the structural join and wet assembly is commonly used. This does not make for a good electrical connection and so at any place where one section of structure is bolted or rivetted to another and it has to carry current then a dedicated bonding lead across the joint is called for.

It is importand to get this right as you will suffer all sorts of EMI/EMC problems during service if it's not.

 

[jstolp]

Thanks,

Your reference to Mil-B-5088 was exactly what I needed. Also, I looked up the specification for wire braid (A-A-59569A) and found that for each braid size there was a corresponding current rating and AWG equivalent. End of problem. Your reminder on proper installation techniques was appreciated.

 

[Meggsy]

If you are working on an installation that has a PT6A engine I would strongly recomend that you read this article on ESD damage to the bearings in the engine that has caused a number of in flight shut downs

http://www.casa.gov.au/avreg/aircraft/AWB/24/001.htm

I think you will find it very intersting
Regards Ian