In intrinsically safe concept, we know that there is a limit for energy released in the hazardous location in case of a short circuit between transmitter's wires, for the worst case this limit is 20uJ. Nothing is said about the transmitter's power. Think that we have a 1 watt transmitter connected to the IS circuit which satisfy the above limit. 1 watt means the transmitter is consuming 1000,000uJ each second. Most of this energy is converted to heat at the transmitter.

Can anyone explain why this 1 watt transmitter which is delivering 1000,000uJ to the environment is not breaking the 20uJ limit of IS circuitry?

 

An IS transmitter is usually a passive component and cannot release more than a very low amount of energy back into the external circuits. All a 4-20 mA transmitter does is to modulate the resistance presented to the terminals so that the current in the circuit is held to the appropriate value.

If a transmitter does contain internal energy sources such as batteries, or has a separate power supply, then the terminals feeding the external circuit must be protected by energy limiting circuits in the same way as a powered circuit in the safe area.

 

When the data sheet of an IS transmitter says its power consumption is say 1 watt, it means the transmitter is consuming 1000,000 micro joules per second most of which is converted into heat at the transmitter. This is confusing to me as I expect that it should not be allowed to release more than 20 micro joules to the hazardous environment.

 

In an IS system, the concern is with electrical energy - not thermal. As long as the surface temperatures of any parts exposed to the flammable material are below the ignition temperature of the material, there isn't a problem.

A typical transmitter drawing 25 mA (allowing for a bit extra to indicate a fault condition) with a 25 VDC supply and 250 ohm load resistor will drop a bit less than 20 V - so its power dissipation will be 0.5 W. In most cases, this will lead to an undetectable temperature rise of the transmitter surface.

 

I conclude that in case of having a problem in an IS transmitter, say a loose short circuit in the transmitter due to a transistor failure which leads to conversion of this 0.5 watt to heat and raises the surface temperature of that transistor above the ignition temperature of the surrounding gas, this will cause an explosion although the transmitter is connected to IS barrier. Is that correct?

 

In theory, yes. But you have also to take into account the probability of such a failure, and the likelihood that the temperature would reach a level likely to cause a problem. Remember that the maximum power available to the transmitter is restricted by the energy limiting features of the power source - so the maximum energy is less than that required to ignite the flammable material.

This energy could conceivably all be converted to heat in the event of some internal fault - but in general the nature of faults inside IS equipment is controlled as part of the specification, for example Ex ia equipment must remain safe in the event of two independent internal faults. So the probability of a fault leading to a dangerous condition is very low. Then that level of thermal energy has to raise the exposed surface temperature to above the ignition temperature of the material. The temperature rise will be affected by a number of conditions such as the ability of air to circulate around it, but in most cases convection will easily deal with any heat released, without excessive temperature rise.

You could try an experiment - take a transmitter housing, install a resistance element inside it, and apply the required amount of power to the element. Then measure the transmitter surface temperature.