Ground lead should only conduct in error state

Hello All, I was always taught that the 3 wire in a typical power plug was for protective earth. It was only meant to conduct during a problem such as a wire chafing to case and risking shock to operator. I have been working in medical imaging many years and have never encountered a machine that used the ground in normal operation. Here is what happened. Working on a Philips IE33 Ultrasound machine. Had the power plug plugged into a safety tester and intentionally opened the ground. At that moment it stopped transmitting computer image (TCP/IP) connection. Did it over and over to prove that with the ground leg open the TCP/IP connection would stop. At first I thought maybe the design folks intended it but that wasn’t true due to the fact that it still acquired images. So why would the use the ground in part of the circuitry when its only there for safety purposes? Please help It really bothers me. Thanks in advance, Michael

Hi,

Technically, i think what your thinking is an earth ground. Earth grounds are for safety purposes. Mostly they are used to protect from lighting or other threats to the device. The most probable reason that the machine won't work is that it needs the ground for its operation. If we go back to the concept of circuits, you do need a ground for the current to flow through.

You are correct. Mainly used where there is a case made of a conducting material that might go live if a live wire comes into contact with it, which prevents a shock if you are at 'earth' potential, and come into contact with it. That's why doubly insulated cases don't need an earth wire.
The power earth should never be used as a return for the 'ground' side of a signal cable, but it might happen if there is a faulty signal ground and both devices are bonded to the system 'earths'.
Medical equipment has to pass much more stringent tests as well, the X Y capacitors used as filters on the mains can be a problem I believe.
I would have thought that if you are testing the Ultrasound machine, that no other devices should be connected to it anyway, you are doing a safety check on the earth, which may well be affected by any other devices that are plugged into it. The device on the other end of the Ethernet cable should be tested as well, but separately.
If you want an answer, I would check the continuity of the Ethernet cable by using another cable. Also check that you are not getting a transient in the system when you open the ground, which kills the connection. Check that by stopping the transmission before you open the earth, then try and restart it. Without knowing more about the system, hard to say what else it could be.
Cheers,
Richard