Barrier for IS approved instrument


Thread Starter


This is the situation:

There are too many temperature transmitter placed in safe area and they are all approved as IS (intrinsically safe). they have to be connected to Pt100 RTDs in the field placed in hazardous area Zone 1. in this case as the temperature transmitter is approved as IS do we need to place any isolator or barrier between the temperature element (PT100) and temp transmitter placed in safe area?

as another question, while the temp transmitter is certified as IS do we need other method of protection like EXD?

Bruce Durdle

An RTD is a purely passive element and does not (provided there is no capacitance or inductance added to the element) store or generate any energy. Under the IEC codes, it is recognised as "simple apparatus" and does not need certification. You will need however to note this on your documentation for the loops involved.

Re your original question - adding a separate barrier to a general loop using a non-IS component does not make the loop safe. Ever element connected into an IS loop downstream of the energy limiting device must be certified as IS. (even if it is, say, a series indicator on the lop located in a safe area.) There should be no difference between discrete energy limiters and those built-in to equipment provided the entity parameters of each are equal.

Dear Bruce

Thank you for your good explanation, As I understand to make a loop IS we need to have all components connected to the loop certified as IS to have a safe loop, even a simple local indicator placed in safe area. Please correct me if I am wrong.

But I didn't get the answer of my question clearly.

Please imagine the situation that we have a rail mounted temp transmitter in safe area and this transmitter is certified as IS; in the field and hazardous area zone1 we have a pt100 inside an thermowell in this situation do we need to place an energy limiting barrier between the rail mounted transmitter and pt100 or not?

If we need it please explain why? considering when an instrument is certified as IS it means the loop energy is limited.

Bruce Durdle

You do not need to have an energy limiter between the transmitter and the RTD as long as the RTD meets the requirements of "simple apparatus".

However, the loop will not meet IS requirements unless the transmitter is connected to its supply through an energy limiter - even if the transmitter is located in a safe area. This is to cover the situation where some external power source breaks through the transmitter and puts an unsafe level of voltage or current into the temperature element circuit.


Excellent explanation. In fact, you have answered the question in the first reply first paragraph itself by referring to "simple apparatus".

Bruce, i would like to know your expert opinion the situations below.

1. We have 50 Ex-D transmitters installed in zone1 field, with 50 barriers installed inside the safe area for these loops. What is the resultant combination? We cant say that these can be certified for IS because of the transmitters. Right? (Junction boxes are suitable for IS type - Ex e if i remember). Can the loop be certified & signed off as Ok without changing the cables, without changing the JB, without removing the barriers? In this case, can i open the instrument when it is powered up?

Bruce Durdle

Hi Sastry,

Using an Ex d transmitter (or Ex e, or Ex n) on an Ex i loop is probably one of the most common faults in explosion protection - the thinking seems to be that the Ex i barrier protects the wiring and the transmitter must be safe because it's certified - so you get a double layer of protection. This however is totally incorrect..

In fact, the use of a non-Ex i transmitter makes the whole of the Ex i equipment worthless, and the barriers are effectively a waste of money. Unless the transmitters meet the limits on energy storage in their internal inductance and capacitance, they could well act as the source of an incentive spark - and without IS certification you won't have the "entity" parameters to make a decision. This is quite apart from any other issues.

And the use of an Ex d transmitter adds requirements for cabling and glanding for instance that do not apply to Ex i - so the Ex d transmitter installation may not be up to the code requirements, meaning that that type of protection is also invalid.

One solution is to check with the transmitter supplier/manufacturer - in many cases, transmitters are made to a single common design and simply labeled according to the required method of protection. You may be able to simply get a written assurance that the transmitters do meet Ex i requirements and keep the existing installation. You will need to add this information to your site documentation and make sure that the transmitters are not treated as Ex d. You also need to obtain the entity parameters for the transmitters and check that the barriers are in fact suitable for use with them. In this case, you can open the transmitters on-line.

Another is to ensure that the wiring at the transmitters meets Ex d requirements - specifically Ex d-certified glands. You could leave the barriers in place but they are doing nothing for you, and could be a source of confusion later on - my recommendation would be to remove them. However, in this case you need to make sure that there is no mention of Ex i protection associated with the transmitter circuits, and that the wiring is fully segregated from other Ex i loops. If the transmitter cables use blue sheathing to indicate IS you have a problem in that this will need to be changed in some way. As the system will now be Ex d rather than Ex i you need to follow Ex d rules throughout and in particular you cannot use live maintenance.