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COZY: Grounding EFIS Components on Composite Airframe


Andrew Anunson
 

I wrote:
"When installing avionic components (fuel senders, OAT sensors) on a composite airframe, what is the best way to ground the component?"

Garmin support wrote back to me and stated:
"The ground wires needs to go to aircraft ground or using pins 7 and 10 will work."

The Garmin response was specific to the resistive fuel senders, and pins 7 and 10 are two pins in their engine monitoring system that are labeled as "Fuel Qty 1 Gnd" and "Fuel Qty 2 Gnd" but in the manual are only shown to be used as grounds with the capacitive type fuel senders (although the resistive fuel senders showed a need to be connected to aircraft frame ground).


So... 
Since we don't have aircraft ground at the fuel sender, its OK to use shielded, twisted, 2 conductor wire from the sender to the EMS, with the ground connected to ground pins in the EMS.  The EMS is grounded to the central ground point on the firewall.  The shield is connected to the d-sub shell at the EMS (shields typically only get connected at one end of the shield).


Thanks for the ideas,
Andrew Anunson
Cozy MKIV #1273
Pound, VA


KEN4ZZ
 

Hi Andrew-

Good to see you've gotten some definitive guidance from the guys who designed your equipment.  The reason I was hesitant re pins 7 and 10 grew from experience I painfully learned last year (fortunately subject harnesses had not been installed yet) when plowing my way thru my massive panel upgrade.  Especially when replacing analog boxes with digital boxes, it was made abundantly clear to me by various mfg. tech support types that ground was not an acceptable substitute for the low side of digital hi - low pairs.  Except sometimes when it was……  

Made me gun shy enough to call or email if something wasn't absolutely explicit in the relevant schematic.

Ken

On 1/11/2021 9:49 AM, Andrew Anunson via groups.io wrote:
I wrote:
"When installing avionic components (fuel senders, OAT sensors) on a composite airframe, what is the best way to ground the component?"

Garmin support wrote back to me and stated:
"The ground wires needs to go to aircraft ground or using pins 7 and 10 will work."

The Garmin response was specific to the resistive fuel senders, and pins 7 and 10 are two pins in their engine monitoring system that are labeled as "Fuel Qty 1 Gnd" and "Fuel Qty 2 Gnd" but in the manual are only shown to be used as grounds with the capacitive type fuel senders (although the resistive fuel senders showed a need to be connected to aircraft frame ground).


So... 
Since we don't have aircraft ground at the fuel sender, its OK to use shielded, twisted, 2 conductor wire from the sender to the EMS, with the ground connected to ground pins in the EMS.  The EMS is grounded to the central ground point on the firewall.  The shield is connected to the d-sub shell at the EMS (shields typically only get connected at one end of the shield).


Thanks for the ideas,
Andrew Anunson
Cozy MKIV #1273
Pound, VA