why MTL706 barriers cannot work without ground connect?

I have already gone thru the detail sheet about the barrier MTL706, but still [confused] on how this barrier functions? It is stated that without ground connect, the barrier lost his reference to 0V which will effect barrier: not function or funny feedback signal occur ...

appreciate anybody can explain how the circuit works ....

Hi,
This kind of barrier is basicaly a combination of several zeners and resistors that effectively "clamp" the voltage levels at both the +ve and -ve terminals to ensure limited voltage and therfore ignition energy is present in the zone. The zener/resistor network needs a point of reference which is the Earth connection, without a good earth connection barriers cannont function.

Earthing is an important point to pay attention to in Ex areas and should be seriously considered. If you are not familar with how to earth Ex equipment the easiest method for isolation is to use galvanic seperators or isolation amplifiers which do not require earthing like barriers. Before putting an Ex loop into service you should fully understand the design and compliance of each of the components in the loop.

Hope this helps?

Paul...

The reason for the good ground is not to make it work, it's to make it safe. If the barrier is not securely tied to ground there is a possibility that dangerous voltage levels can reach the intrinsic safe side where they might cause a spark.

Your ground should connect directly to the plant main ground, i.e. not through power system ground.

Follow the manufacturer's recommendation and your system will be safe.

Regards,
Roy

In addition to requiring a low impedance, high integrity earth connection for safety reasons, the MTL706 is actually quite a complicated barrier that contains additional circuitry to compensate for the voltage drop that normally occurs across a passive zener barrier. This circuit also requires a reference earth, I believe.

Steve
MTL
syates at mtl-inst. com
www.mtl-blog.com