An XP- is generally heavier than an IS- device because of the inherent safety provision with respect to containing the explosion. However, the additional functionality provided by XP protection means that the operator can complete tasks that are otherwise difficult or even impossible with IS.
For example, the only way to achieve good imagery in explosive environments, when a high energy flash is needed, is to use the XP protection concept.
XP design considerations
From a basic design standpoint, an XP device must be able to withstand an explosion within the enclosure. As briefly described above, escaping gas must be allowed to cool as it expands and passes through the flame paths from inside the enclosure to the outside environment. Flame paths are carefully calculated and manufactured to extreme tolerances in order to achieve the desired cooling effect.
XP devices must of course be serviced by qualified personnel with appropriate tools. Flame paths need to be rechecked regularly in accordance with the certification, using calibrated devices.
A major design requirement for European certified – ATEX – devices is to select and test any non-metallic component for its ability to dissipate electrical energy and hence be termed ‘anti-static’. This is known as surface resistivity testing.
Often overlooked in favour of electrical systems, the anti-static properties of polymer bodies is a huge contributing factor to any device operating in a hazardous area defined by the ATEX Directive. Even mechanical or clockwork devices which may have no electrical components should still be assessed with respect to the effects of static electricity.
Usually, enclosure materials are selected with a high carbon content in order to satisfy the needs of the test standard. However, it is important to note that even though a material manufacturer may state that a product complies with - or is compliant to - a particular requirement, the certification body will demand a representative sample testing in order for it to be acceptable.
When a manufacturer adds certification marks to a device, the call for additional quality control and subsequent third party audits also increases. Typically, for devices certified for use in the US or Canada, audit frequency is once per quarter. For European ATEX certified products, the audit is generally on an annual basis.
A manufacturer of IS equipment cannot automatically manufacture XP equipment and vice versa. Before the device can be manufactured, Quality Assurance Notification, for each applicable certification must also be upgraded, audited and certified.
For ATEX certified equipment, separate Quality Assurance Notification is required in order for the device to be legally sold. This is different from a standard ISO9001:2000 Quality Management System and far more stringent with respect to inspection, testing and acceptance.
IS v XP – which is best?
The answer to this question is obviously a subjective one. There are pros and cons for each protection concept that will dictate the route a manufacturer would take to design, certify and manufacture an instrument.
If a device requires a significant amount of power to operate, the XP must be used. IS can only be used for very low power applications. And if the construction of a device cannot be controlled to a component level, XP must also be applied. Regardless of protection concept, anti-static materials should be used for any non-metallic enclosure and the construction of metallic parts must follow the correct metallurgical breakdown as defined by the governing standard.
IS and XP differ in terms of ATEX classification. ATEX has two types of IS certification namely Ex ia and EX ib. Both are termed IS but they have different end-user applications. EX ia may be used in a Zone 0, Zone 1 or Zone 2 explosive area where EX ib may only be used in Zone 1 or Zone 2.
In layman’s terms, this means that Ex ia has an increased safety aspect with respect to redundancy and as such the device may be used in enclosed spaces where gas or dust is likely to be present at all times. Ex ib is ideal for open areas where explosive gas or dust may prevail under certain circumstances.
Conversely Ex d equipment (explosion-proof) is allowed in Zone 1 and Zone 2 explosive areas which account for the majority of applications for portable equipment.
For explosive areas regulated by classes and divisions, both XP and IS may be used. However, as we’ve already covered, even though a device is termed IS and certified to ATEX, that doesn’t mean its IS certification is sufficient for US standards.
An interesting point however is the 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.
So, in summary, your choice in selecting IS or XP must be based on the functionality of the device itself and the area in which it is intended to be operated. XP however is the best solution for higher power devices. Wherever possible use certified equipment but remember country standards differ. And if you have no choice but to use an uncertified device then all aspects of that product must be assessed including electrical, mechanical and static energy.