I have a somewhat complex circuit controlled by an Arduino, which is supplied by a self powered usb hub (earth grounded 5V supply). A part of the circuit has to be powered by 12-20V DC.

When I plug in a earth-grounded supply to provide 12+V, everything works as expected. When I provide the 12+V through a floating supply, instead, and then connect the Arduino's usb to the hub, sometimes the hub 'bricks' and I have to disconnect and reconnect it to make it work again. Arduino's ground (hence the hub's ground) and the ground of the other supply are joined in a single ground point. I am absolutely sure I'm not exceeding any of the Arduino/power supplies/hub current ratings.

So my question is, is it possible that connecting a floating supply to an earth grounded circuit, when the former is already powered on, could cause some voltage spike/imbalance or wreak havoc in the entire circuit?

Any help appreciated :]

P.S. A super-simplified schematic to show the situation

^{simulate this circuit – Schematic created using CircuitLab}

I have a somewhat complex circuit controlled by an Arduino, which is supplied by a self powered USB hub (earth grounded 5V supply). A part of the circuit has to be powered by 12-20V DC.

When I plug in a earth-grounded supply to provide 12+V, everything works as expected. When I provide the 12+V through a floating supply, instead, and then connect the Arduino's USB to the hub, sometimes the hub 'bricks' and I have to disconnect and reconnect it to make it work again.

Arduino's ground (hence the hub's ground) and the ground of the other supply are joined in a single ground point. I am absolutely sure I'm not exceeding any of the Arduino/power supplies/hub current ratings.

So my question is, is it possible that connecting a floating supply to an earth grounded circuit, when the former is already powered on, could cause some voltage spike/imbalance or wreak havoc in the entire circuit?

P.S. A super-simplified schematic to show the situation:

^{simulate this circuit – Schematic created using CircuitLab}

I have a somewhat complex circuit controlled by an Arduino, which is supplied by a self powered usb hub (earth grounded 5V supply). A part of the circuit has to be powered by 12-20V DC. When I plug in a earth-grounded supply to provide 12+V, everything works as expected. When I provide the 12+V through a floating supply, instead, and then connect the Arduino's usb to the hub, sometimes the hub 'bricks' and I have to disconnect and reconnect it to make it work again. Arduino's ground (hence the hub's ground) and the ground of the other supply are joined in a single ground point. I am absolutely sure I'm not exceeding any of the Arduino/power supplies/hub current ratings. So my question is, is it possible that connecting a floating supply to an earth grounded circuit, when the former is already powered on, could cause some voltage spike/imbalance or wreak havoc in the entire circuit?

Any help appreciated :]

P.S. A super-simplified schematic to show the situation

^{simulate this circuit – Schematic created using CircuitLab}

I have a somewhat complex circuit controlled by an Arduino, which is supplied by a self powered usb hub (earth grounded 5V supply). A part of the circuit has to be powered by 12-20V DC. 

When I plug in a earth-grounded supply to provide 12+V, everything works as expected. When I provide the 12+V through a floating supply, instead, and then connect the Arduino's usb to the hub, sometimes the hub 'bricks' and I have to disconnect and reconnect it to make it work again. Arduino's ground (hence the hub's ground) and the ground of the other supply are joined in a single ground point. I am absolutely sure I'm not exceeding any of the Arduino/power supplies/hub current ratings.

So my question is, is it possible that connecting a floating supply to an earth grounded circuit, when the former is already powered on, could cause some voltage spike/imbalance or wreak havoc in the entire circuit?

Any help appreciated :]

P.S. A super-simplified schematic to show the situation

^{simulate this circuit – Schematic created using CircuitLab}

I have a somewhat complex circuit controlled by an Arduino, which is fuelled by a self powered usb hub (earth grounded 5v supply).
  A part of the circuit has to be powered by 12-20V DC. When I plug in a earth-grounded supply to provide 12+V, everything works as expected. When I provide the 12+V through a floating supply, instead, and then connect the Arduino's usb to the hub, sometimes the hub 'bricks' and I have to disconnect and reconnect it to make it work again. Arduino's ground (hence hub's ground) and the ground of the other supply are joined in a single ground point. I am absolutely sure I'm not exceeding any of the Arduino/power supplies/hub current ratings. So my question is, is it possible that connecting a floating supply to an earth grounded circuit, when the former is already powered on, could cause some voltage spike/imbalance or wreak havoc in the entire circuit?

Any help appreciated :]

P.S. A super-simplified schematic to show the situation

^{simulate this circuit – Schematic created using CircuitLab}

I have a somewhat complex circuit controlled by an Arduino, which is supplied by a self powered usb hub (earth grounded 5V supply). A part of the circuit has to be powered by 12-20V DC. When I plug in a earth-grounded supply to provide 12+V, everything works as expected. When I provide the 12+V through a floating supply, instead, and then connect the Arduino's usb to the hub, sometimes the hub 'bricks' and I have to disconnect and reconnect it to make it work again. Arduino's ground (hence the hub's ground) and the ground of the other supply are joined in a single ground point. I am absolutely sure I'm not exceeding any of the Arduino/power supplies/hub current ratings. So my question is, is it possible that connecting a floating supply to an earth grounded circuit, when the former is already powered on, could cause some voltage spike/imbalance or wreak havoc in the entire circuit?

Any help appreciated :]

P.S. A super-simplified schematic to show the situation

^{simulate this circuit – Schematic created using CircuitLab}