The Temperature Confusion Of The ATEX Marking Scheme Explained
Of all the confusion generated by the cryptic ATEX marking scheme, often the worst is caused by the different temperature ratings applied to a unit.
The first thing to realise is the difference between ambient temperature and maximum surface temperature.
Ambient temperature is the easiest – that’s the highest external temperature touching the unit – be it air temperature, temperature of what it’s attached to or the heat of the sun warming the unit. By default ATEX assumes -20°C to +40°C, but often manufacturers will apply extended ratings which will be written as (-20°C < Ta < +60°C).
Surface temperature is harder – not least because of the counterintuitive numbering scheme! Any piece of electrical equipment dissipates power, even if it is just a wire or a connector block. This power creates a temperature rise. Add this temperature rise onto the ambient temperature and you get a maximum surface temperature.
Why does this matter? Gases will auto-ignite at different temperatures. Less volatile gases such as hydrogen can come into contact with temperatures up to 536°C before risk of ignition. In the event of a gas such as Carbon Disulphide being present then it only takes temperatures of 90°C to cause ignition.
So, onto this numbering scheme – there are a set of surface temperatures split into so-called T-classes, T1 to T6. T6 is the most universally applicable covering surface temperatures lower than 85°C. T1, at 450°C requires that the user knows that the most volatile gases won’t be present on site. Custom T marks are also possible, e.g. T120 would denote a 120°C max surface temperature.
As you have now seen, the T-class is a product of the maximum power dissipation in an electrical product, added on to the maximum ambient temperature. If as a user you have a high ambient temperature, such as a 140°C wellhead then you are already limited to the T-classes of the equipment mounted to it. From the other point of view, as a manufacturer, if you make something with a high ambient temperature to suit many applications you will be limited if customers require low temperature T-classes. This is the reason why some products will carry different marking options.
Author: Ben Brooking
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