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‘IS’ or ‘XP’ that’s the question

In the field of hazardous area operation the terms ‘intrinsically safe - IS - and explosion proof – XP - automatically spring to mind. But, what is the difference between the two and which is the most appropriate for your application? EP&A finds out…

Starting with the basics, Intrinsically Safe is a protection concept based around limiting the available electrical energy so that sparks cannot occur from a short circuit or failure and cause the ignition of an explosive atmosphere.

With an IS system, the entire power of a device is controlled. This includes, but is not limited to, the batteries. Hence, inductive and capacitive loads are also assessed to the extent that they cannot cause a spark. These constraints, however, would preclude the use of many electrical instruments in hazardous areas that normally operate in a high power range. So, if the instrument you are considering falls into this category your focus needs to be on the explosion-proof XP protection concept. Quite simply, if power is an issue, think XP

The CorDEX Centurion XP Dual digital camera provides a good example. It is believed to be the only ATEX certified explosion proof digital camera available for hazardous above and below-ground environments that enables the capture of clear images at distances of between 50 and 100mm with a standard macro lens. A typical use would be the documentation of detail on equipment rating plates. This enhances the scope of the camera, adds value to the inspection and allows visual records to be more comprehensive. Thanks to its IP rating, this British designed and developed instrument can offer benefits to the hazardous environment.

An important benefit of the camera’s design is a high intensity strobe flash that enables illumination for still shots as well as a detachable low intensity LED lamp for close-up shots and movie capture. Its high energy flash could not, however, be part of an IS device because it is powered by a capacitor that has a discharge greater than that accepted by the IS standard. The alternative solution is an IS digital camera with no strobe flash facility but whose imagery in low light situations would be extremely poor. The trade-off is clear.

By using XP as a protection concept, relatively high power devices, such as the camera flash, can be safely used in an explosive area. The Centurion digital camera is designed and tested to contain any explosion which may occur. It channels the expanding gas into the outside world via calculated and constructed flame paths. As the gas travels through these flame paths, it cools, preventing ignition of the external atmosphere and protecting the operator and the facility from harm.

In common with all XP devices, the camera must be serviced by qualified personnel with appropriate tools. Flame paths need to be re-checked regularly in accordance with the ATEX certification, using calibrated devices.

ATEX classification

IS and XP differ in terms of ATEX classification - the European certification standard for hazardous area operation. ATEX has two types of IS certification designed for different end-user applications - EX ia and EX ib. Ex d applies to explosion-proof equipment and covers the majority of applications for portable equipment. Instruments holding this certification are deemed suitable for use in open areas where explosive gas may prevail under certain circumstances.

An interesting point is that flame path gap requirement for ATEX compliance is 100% larger than that required for US certification. As the size of the ATEX openings is greater, the gas takes less time to cool, resulting in a product engineered to a higher degree of safety than is required in the US.

When a manufacturer adds certification marks to an instrument, the call for additional quality control and subsequently third party audits also increases. For the US or Canada, a quarterly audit is required but for cameras with European ATEX certification, the audit is generally on an annual basis.

An important consideration is that for ATEX certified equipment a separate Quality Assurance Notification is required in order for the instrument to be legally sold. This is different from a standard ISO9001:2000 Quality Management System and is far more stringent with respect to inspection, testing and acceptance.

So, in conclusion, which is the best, IS or XP? The answer is subjective. There are pros and cons for each protection concept but, if an instrument requires a significant amount of power to operate, then XP is the clear choice.

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