Engine Grounds

[Jim P]

sI have a couple questions on engine grounds...

1) I have the option of grounding the engine to either the firewall ground block or the negative post of the battery. Is there a preference? I have about a 12" #2 AWG from the ground block to the negative battery post. Technically, I wouldn't think there's much difference.

2) Where are most grounding the engine? It's getting a bit crowded and I'm looking at one of the accessory case bolts close on the right rear of the engine. I can also easily take it off one of the pad studs for the SD-8.

Thanks,

Jim

[pierre smith]

Hi Jim,
We used a very flexible braided stainless cable from the right rear of the case to the battery nearby. I think either way is fine but it is very necessary.

Regards,

[N941WR]

Jim,

You need to do both a ground from the engine to the airframe and from the battery negative post to the airframe. My preference was to have only one wire to the battery post.

This picture shows how I did it.


Some people even add a secondary case to airframe ground just incase the primary breaks. Since I have braded stainless steel oil and fuel pressure lines, that is my secondary ground path.

[carguy614]

I am with Bill...Aux ground can't hurt. If using EMS, ground potential is critical to get accurate oil temp readings. I used 2 gauge to case ground to carry starter loads, as per usual. Installed braid from firewall ground point to negative cable, and braid from firewall point to accessory case as a redundant. Maybe overkill.
Regards...Chris

[Jim P]

Thanks guys. I'll do the firewall - accessory case route. I already have stainless lines as well so that's a bit of redundancy as well.

Jim

[az_gila]

A lot of certified planes seem to have an extra ground strap/braid from the engine to the motor mount.

I guess this is a redundant path, if the main direct engine to airframe bond fails. In our case, the engine mount is well grounded to the basic airframe via the 6 mounting screws, so it like a good idea for a "back-up" ground...

Has anyone done this on a RV?

gil A

[Steve Sampson]

What are peoples thoughts on WHERE to attach to the engine?

One location is an empty hole just aft of the dip stick. Curiously it is bigger than an AN4 but does not permit an AN5?

Comments?

PS Someone mentioned the steel brade on the oil & fuel lines. Surely the logic of two grounds is to protect these from ever carrying the starter current? If they do, is that not a good way to start a fire since there is surely not enough 'meat' on them?

[N941WR]

Originally Posted by az_gila

A lot of certified planes seem to have an extra ground strap/braid from the engine to the motor mount.

I guess this is a redundant path, if the main direct engine to airframe bond fails. In our case, the engine mount is well grounded to the basic airframe via the 6 mounting screws, so it like a good idea for a "back-up" ground...

Has anyone done this on a RV?

gil A

Gil, I have seen this done a number of times. The secondary path is terminated to a hole drilled in a gusset on the -6, -7, or -8 mount and held in place with an 8-32 bolt & nut. Of course, you need to remove any powder coating. (BTW, the -9 doesn't have these gussets on its engine mount.)

The ground strap/braid from the engine to the airframe or engine mount is the primary load path. This returns the current to the storage unit aka battery during startup and operations.

A secondary path is a "nice to have" in the event the primary path fails such as if a tab breaks off.

The engine mount can be used to carry that primary and secondary path, if you chose. Typically I see the primary ground go directly to the fuselage and the secondary go to the engine mount. BTW, make sure you don't terminate either the primary or secondary path at the same location just incase a failure at that connection doesn't compromise both paths.

Originally Posted by Steve Sampson

What are peoples thoughts on WHERE to attach to the engine?

I picked up one of the vacuum pump bolts for my primary.

Originally Posted by Steve Sampson

PS Someone mentioned the steel brade on the oil & fuel lines. Surely the logic of two grounds is to protect these from ever carrying the starter current? If they do, is that not a good way to start a fire since there is surely not enough 'meat' on them?

The idea of the secondary path is to keep you running, should the primary path fail. It is not intended to carry a starting load and I suspect most secondary paths would burn up under a starting load, transferring the current to oil and fuel pressure stainless steel lines. BTW, most secondary paths use a smaller wire, say 12 AWG. Again, the idea is to keep the engine running and all the engine gauges working (EGT, CHT, etc.).

It is unlikely that using the oil and fuel pressure lines as a secondary ground will cause a fire as the electricity goes around the oil and fuel and not through it. Just my opinion. (Kind of like the guy who repairs high tension lines by climbing from a helicopter doesn't get knocked out by the static charge.)

[Steve Sampson]

Originally Posted by N941WR

the idea is to keep the engine running and all the engine gauges working (EGT, CHT, etc.).

It is unlikely that using the oil and fuel pressure lines as a secondary ground will cause a fire as the electricity goes around the oil and fuel and not through it.....

Since you would be unaware the primary path was broken my concern is the next time you started the engine on the ground. Wooosh!

[N941WR]

Originally Posted by Steve Sampson

Since you would be unaware the primary path was broken my concern is the next time you started the engine on the ground. Wooosh!

Steve,

See the last part of my prior post, I doubt that will happen.

Besides, say you have two dedicated, heavy grounds, and one breaks. You wouldn't know it. Then, say two weeks later, you still haven't removed the cowling and the second one breaks. The next time you go to fly and "wooosh"? Probably not.

I have heard of people melting their wiring when trying to start an engine w/o the primary ground in place but never starting a fuel fire.