Power Supply for Intrinsic Safety System

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Thread Starter

Ashley Dawson

The Australian Standard on intrinsic safety contains the following:

3.6.2 Restrictions on Safe Area Equipment
...
If the equipment is mains-fed, it shall be fed through a suitably fused, double-wound transformer
...

Does this mean that swictch mode power supplies cannot be used to supply an IS circuit? If so, what is the advantage safety-wise of a double-wound fused transformer as opposed to a fused switchmode power supply?
 
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Curt Wuollet

A simple 1:1 isolation transformer would solve the problem unless it needs to delineate the "barrier". Further, this could be a single transformer for multiple IS ckts unless I read it wrong. It's not a bad idea in any case and is pretty much standard when the plant power is three phase.

Regards

cww
 
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Bruce Durdle

Hi Ashley,

If you have a semiconductor component between your mains supply and the power supply outlet, and this component fails to a short-circuit, you will apply full mains voltage to the inputs of your IS gear. It is a requirement that any component cannot fail in such a way as to apply
excessive voltage to the barriers.

If you are using AS2380.7(?) note that the current draft version of AS/NZS2381.1 states that this standard will be withdrawn as from 28 February 2005 - its replacement is AS/NZS 600789.11.

Bruce
 
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Philip Brown

Yes it is best to use AS/NZS 60079.11 as it is the latest standard for intrinsically safe apparatus in Australia.
You may use any power supply topology you like provide it meets all of the requirements set forth in 60079.11. The transformer, mains frequency or switch mode must have 1mm minimum solid insulation between the primary and secondary, have a fuse in each non-phase conductor of the primary. The fuse must have an interupting or rupturing capacity of 1500 amps at 250 VAC. The transformer should have a thermal fuse embedded within the windings that limits any fault temperature rise to a level that can be withstood by the insulation materials used within the transformer. The transformer shall pass several type tests.
The turns ratio of the transformer will be used to assess the output voltage. The primary fuse rating x 1.7 and divided by the turns ratio will be used to assess the available output current.
The output of the transformer may not be intrinsically safe until an infallible current limiter and infallible voltage limiter are added to the output.
If the transformer incorporates a screen between the primary and secondary, the thickness of the copper screen will determine the maximum rating.

The comment posted about applying excessive voltage to barriers is misleading. If a correctly rated and certified zener barrier is used between the safe and hazardous areas then you can connect 250VAC to the safe area terminals without fear of this voltage reaching the hazardous area. The fuse in the barrier will rupture and fail safe.

Philip Brown
 
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What if the primary of the transformer is an intrinsic safe circuit? My understanding is the fuse can be replaced by rating the transformer at 1.5x. Also standard is not clear whether the construction requirements still apply (separations per table 5 between the windings).

Thanks
 
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