T Rating for Transformers, Ex rated equipment

[IsLimiter]

I am currently shopping for a 240V/240V Isolation Transformer, dry type, approx 500VA to 2KVA. The application is for instrumentation panels to be located in a Zone 2 classified area. As you can already tell this is an IEC job, so in order for the entire panel to be ATEX rated, each component must be Ex rated itself for Zone 2 to certify the entire panel. What I have discovered thus far is that control power transformers are not Ex rated themselves as there is a very low potential to create a spark (no moving parts). So what I suspect is a transformer should be specified with the appropriate T-rating to ensure that under normal operating conditions, the surface temp of the transformer will not exceed a certain flashpoint. Could anyone please provide information on how to select a transformer with the appropriate T-rating that will allow a panel to be certified for Zone 2 classified area?

 

[7anoter4]

European standard EN 50014 requires that the maximum surface
temperature be subdivided into six classes from T1 to T6,
assuming a reference ambient temperature of 40°C. Should the
reference temperature be different, this temperature must be specified on the respective instrument.
T1 450dgreeC;T2-300;T3-200;T4-135;T5-100;T6-85
The minimum ignition temperature of an air/gas mixture is the
temperature at which the dangerous mixture ignites without
electrical energy being supplied.
This parameter is important because it establishes the maximum
surface temperature allowed for devices located in a hazardous
location, under both normal and fault conditions. This must always
be lower than the ignition temperature of the gas present.
The "T"-class of a piece of equipment can be compared to the auto-ignition temperature of gases that may come into contact with it and hence a decision can be made as to the suitability of the equipment to be used in that area.If the auto-ignition temperature of the gas is above the T class then this will be the clsass.For instance the auto-ignition temperature of Hydrogen is 500 then its class is T1[450].See:

http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html

 

[IsLimiter]

Ok so what your telling me is I need to find out the type of gas that may be in the area (Zone 2; Low risk, possibility of leakage not likely but possible) and this will dictate the T-Rating. This makes since but I have been shopping for transformers and have never seen a T-rating in any specifications. Where can I find transformers with a specified surface temperature limit?

 

[ScottyUK]

You might want to have a look at the EN 600079 series of standards and read the one(s) applicable to your chosen method of protection against explosion. I'm guessing for an instrument box you're looking at Ex'e' or Ex'n', or maybe Ex'p'.


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If we learn from our mistakes I'm getting a great education!

 

[7anoter4]

First of all I agree with ScottyUK. If, for instance, the transformer enclosure will be IP00 or IP23
a such dry type control transformer is an open air-cooled transformer, so the maximum
temperature will be in the winding. If the wire insulation is class F-as usually- the maximum permitted temperature will be 155 oC as per IEC 34-1[see]:

http://www.aps-systems.ch/en/Waermeklassen.htm

If the enclosure will be IP-54 then the temperature will be lower[less then 80 oC-usually].
The problem will be in the insulation failure case when the short-circuit in the winding should
rise the temperature at a very high-level. So the enclosure have to be suitable and not an open type. An ex"n" type will be suitable, I think.
In NEC world, the equivalent to Zone 2 it is Class I Division 2.According to NEC art 501 if no
sparking device is involved then a general purpose enclosure is permissible.