I2C; power protection; multiple devices

I have most things tied together over I2C. This includes a Particle Electron (as Master). There is a level shifter between the Electron and a SparkFun RedBoard (Arduino Uno) and an Arduino Mega 2560. I can post a proper Fritzing wiring diagram later. The Electron sits on the Relay Shield.

The two arduino devices did not yet have power.
Thank goodness I had a 5A fuse in my circuit or I might have cooked the Electron. I have two spares. It took me a little bit to discover the 5A fuse was gone – I thought I had cooked the Electron. Fortunately, everything is ok. But I don’t quite understand where 60W (5A@12V) was apparently being pulled through the Relay Shield when the fuse blew.

Applying power to the Arduino devices and then the Relay Shield/Electron in that order is operating but I feel is not completely safe.

What I would like to see is if I apply power to one specific device, that it does not back-feed to another device. I am open to suggestions on how best to accomplish this?

My initial thought is placement of diodes on the 5V and 3V3 supply lines? There are may types of diodes, is there a particular type that I should use?

Here is a rough description of what I have going on:

The level shifter is a SparkFun BOB-12009 with pull up resistors already in play.

Electron -> Level Shifter
  3V3 -> LV 
  GND -> GND
  D0 -> LV1 (SCL)
  D1 -> LV2 (SDA)

Other 3V3 compatible devices are just added to LV1, LV2 but having different I2C addresses. All the GNDs are tied together.

I am using the dedicated SCL and SDA lines on the Mega2560 and Uno. I am only using the 5V line from the Mega2560 to the level shifter.

 Mega2560 -> Level Shifter
   5V  -> HV  
   GND -> GND
   SCL -> HV1 (SCL)
   SDA -> HV2 (SDA)

The Uno’s SCL and SDA are plugged in to the same SCL and SDA of the Mega2560 and the GNDs are all tied together. Other compatible 5V sensors are also plugged into respective SCL and SDA lines.

Everything works as I can find the Electron’s I2C power management chip at 0x36 and all my other devices.

I just would like to solve the apparent power sharing. Reference to a larger project.