I just got this from someone who found it on a US site:
The G500 STC for the DR400 has an option to have the system operated by a switch so you don’t have it connected to the electrical system for engine start.
However the standby attitude indicator is connected directly to the main aircraft DC bus bar.
I’m just wondering what kind of reason should warrant such a requirement?
With a tachometer it is obvious – you want it to work when the engine starts.
Admittedly such an instrument will still stop working if you turn off the electrics master during flight, and that is reportedly a popular tactic by renters to reduce their billing 
But it is less obvious for an engine monitor which does only CHT and EGT.
In the meantime no avionics installer has made any input here and my enquiries elsewhere have not produced a response, so maybe the requirement is found in the STC IM only i.e. the STC won’t be approved otherwise.
Peter wrote:
Who installed it?
Mine was fitted by Mike Walker (retired BA Avionics Engineer), at the same time as other new avionics were installed. He did a good job. Everything worked first time after a fairly complex installation.
Peter wrote:
What I was wondering is whether there is a regulation requiring this type of wiring.
Maybe, I have no idea. I’m just wondering what kind of reason should warrant such a requirement?
This wiring instruction appears in a number of installation manuals.
For example, the Sandel SG102 AHRS gyro can be wired direct, per the IM, so that it is halfway to having stabilised by the time you get around to switching on the avionics and sorting them out. Mine wasn’t done that way because it does expose it to the starter motor spikes, and I judged that to be not worth the risk.
What I was wondering is whether there is a regulation requiring this type of wiring.
My CGR-30P also comes on with the master. It is needed at start-up as it is the only engine instrument, including tacho.
Mine was installed in the Netherlands, but I seem to recall this is how it is designed to be installed according to the manual supplied, so I would expect anyone to do it like this.
My engine analyser (EI CGR-30P) comes on with the Master Switch. It does have RPM, Manifold Pressure and Tacho.
It is also connected to the fuel flow system and records fuel used at Start Up before the Avionics Master is on. It also communicates (RS232) with the IFD550 to give fuel usage and range info on the map display and Datablock.
Interesting that none of the certification specialists here found it (I believe wigglyamp has left) but eventually I did after a lot of digging.
The mandatory equipment is listed in the POH under Weight & Balance!
At the very end of the list is this
which shows that not only is a CHT gauge not mandatory on the TB20GT (“post Mod 151” is what they call the GT) but the EDM700 is not mandatory either.
The tachometer is mandatory (R = required equipment) so a replacement needs to be either PMA or STCd as Primary. But the EDM700 is optional, which might surprise a number of avionics shops who have installed other instruments, saying a Primary STC is required.
The remaining question is what happens when the “Standard equipment” Alcor CHT/EGT gauge is removed, which was done by Socata. I believe it looked something like this
It still doesn’t look like the replacement needs a Primary STC.
It would be interesting to see the corresponding POH page from a TB21 POH. It should say the above gauge is (R) i.e. Required equipment i.e. mandatory.
The other useful thing about that POH section is that it lists a lot of OEM part numbers, not just the anonymised Socata ones
