Does the "T" class of hazardous area equipment (e.g. T4, T5, T6) define the maximum temperature of the surrounding environment in which the equipment can be installed? For example in the case of T4 is the maximum ambient temperature 135ºC?
We have been asked this question a number of times recently and the answer is "No".
The "T" class is interpreted in one of two ways:
The maximum surface temperature of the apparatus - In the case of T4 the maximum surface temperature of the apparatus must not exceed 135ºC degrees C.
The auto ignition temperature of a particular gas - The auto ignition temperatures of all the gasses in group T4 will be greater than 135ºC and less than 200ºC.
The contents of the apparatus will increase the surface temperature of the apparatus when power is applied; during both normal operation and under fault conditions.
As an allowance needs to be made for this temperature rise, the maximum ambient temperature (T amb) of the apparatus can operate in also needs to be defined.
This maximum ambient temperature e.g.+ 40ºC or + 80ºC, will therefore be lower than the temperature defined by the relevant T class, e.g. 135ºC (T4).
A given piece of electrical equipment will therefore be approved for a stated range of ambient temperatures in which it is safe to operate.
Normally this would be T amb -20ºC to +40ºC, unless otherwise stated on the product certification.
K Controls can offer certain products with a T amb as low as -60ºC or as high as +120ºC.
In summary you will need to know the T class and the T amb (maximum and minimum) in order to specify the correct equipment.
(For further information on the T class please click here).
Is specifying "simple apparatus" as simple as it seems? Using micro switches in intrinsically safe circuits to monitor valve position is common practice. A micro switch is defined as "simple apparatus" so it does not generate or store energy above defined limits. Therefore, if it is incorporated into an intrinsically safe circuit, there will be no unexpected sparking that could cause ignition of a hazardous gas or dust cloud.
So far so good, but the switches are of no use on their own. They have to be operated, adjusted, terminated and environmentally protected. In order for this to happen they usually need to be fitted in a "switch box". For intrinsically safe applications some switch box suppliers offer their standard non certified product, complete with "simple apparatus" micro switches. In such cases the safety focus is on the switches and not on the switch box. Closer examination of the switch box prompts one or more of the following questions to be asked:
How can the build-up of electro-static charge be avoided on plastic parts that are likely to rubbed or cleaned in service? Is it enough to simply add a label stating "Do not wipe with a dry cloth" ? Enclosures are sometimes made of plastic and a clear plastic is often used in the construction of the visual position indicator.
How can one be sure that the magnesium content of a light alloy enclosure, in which the switches are fitted, does not result in sparking on impact?
Is the distance between each circuit in the enclosure sufficient to avoid arcing between them?
Is the distance between the terminals and the inside of the enclosure sufficient to avoid arcing across the gap?
Has the continuity to earth of each circuit been tested?
Is the product cleared marked with the intrinsically safe safety parameters and a traceable label?
Is documentation readily available to the user that will assist with the risk assessment required by the ATEX Directive?
If your switch box manufacturer or supplier cannot answer these questions to your satisfaction then perhaps specifying "simple apparatus" is not as simple as it seems.
An alternative would be to specify a switch box that has overall Exi certification to ATEX and or IECEx. This gives rise to another question. ATEX and IECEx are both certification schemes for electrical equipment in hazardous areas, but why are some products dual certified?
ATEX is accepted outside Europe to some degree but its prime objective is free trade within the EU. This is achieved by establishing a level of safety acceptable to all the European national authorities.
IECEx on the other hand was designed as an ISO/IEC Type 5 Certification Scheme administered by a single third party. It uses recognised practice to achieve a common acceptable level of safety. The scope of the Scheme is defined by the standards issued by IEC Standards Committee TC 31. There are currently 34 participating countries with a further 12 countries observing.
The only ATEX document in the public domain is the manufacturer’s “Declaration of Conformity”. This is not the case with the IECEX scheme. Assessment and Test reports are available online and in real time, giving users access to much more detailed information.
This transparency together with compliance to clearly defined standards that are based on recognised practice are leading to the adoption of the IECEx scheme in an increasing number of countries outside Europe. Safety may be the prime objective of IECEx certification but as a single global standard moves closer to becoming a reality, free trade will be one its welcome consequences.
John Rezabek explains how "Dynamic arc recognition and termination" (DART) can deliver dramatically increased power for automated valve networks with the entire circuit meeting IS requirements from the control house to the field device.
To view this article at ControlGlobal.com please click here.
If you would like further information on DART Technology please click here.