Zener safety barriers

Theboss

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Join Date
Jun 2010
Location
Durban
Posts
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Good day, could someone assist me if I'm wrong.
We currently using MTL zener barriers on our current input loops.
I'm measuring 17.5 vdc between earth and 0 vdc of the field side.
Isn't that a concern?
 
I'm not sure.
According to the schematic the zener diode is forward biased and will conduct current through it, hence the voltage drop across terminal 3 and earth... My concern is, if there is a fault? How does the zener diode conduct the fault down to earth, if I already have 17.3 vdc on it?
 
Hold on. Terminal 3 is the negative terminal of the instrument, but it's where the voltage should be positive.

Did you see the manual?

The Zener barrier is there to reduce the available current to the output... not shorting to ground.

The Zener barrier itself, with the ia rating, is made to work safely with component faults inside it too, hence why the schematic shows two Zener diodes (in the manual, not the barrier itself).
 
Terminal 3 is on the barrier and is labelled negative, which should be going to negative of Instrument aswell. Please share manual. Can't seem to find this specific one
 
Which schematic you looking at in the manual?

How did you determine that it's limiting the current? I'm under the impression that the resistor is the current limiter and zener diode limits the voltage. Please explain.
 
Last edited:
Figure 2. The one that says MTL706 basic circuit.

The whole point of these barriers, be it galvanic isolation or Zener, is to limit the current allowed into a hazardous area in order to prevent a spark from ocurring.

Here's a couple of links about the subject.

https://www.mtl-inst.com/images/uploads/EATON_WP_Ex_ic_171016.pdf
https://www.bartec.de/en/downloads/safety-academy/ex-protection.pdf


By the way, if you are not familiar with hazardous area equipment, you shouldn't be touching it.

Are you trying to fix a problem?
 
I'm trying to figure out the flow of current during a fault and normal conditions...

We do have a hazardous plant, that's how I came across these barriers.
 
You right about it reducing the current, but when a fault exists, what happens? For instance if the terminals in the field short each other or one of them down to earth.
 

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