27 July 2010
Weâ€™re all familiar with the terms â€˜intrinsically safeâ€™ and â€˜explosion proofâ€™, but what is the difference between the two and what is the most appropriate for your application?
Let’s start with the basics. Products destined for use in an explosive area may be intrinsically safe (IS). IS 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.
A typical IS circuit, found in a fixed process control instrument, comprises a permanently located device within the explosive area which, in turn, is protected by a safety barrier located in the safe area. These barriers usually incorporate a series of diodes, resistors and fuses that are arranged and sized to limit the energy provided to the device in the hazardous area.
Interestingly, the inclusion of a barrier in the control loop does not mean that any device can be connected downstream. Even though the power may be limited, the device in the explosive area must also be designed to comply with the requirements of the local regulatory standards.
There are many international regulations and requirements which dictate how products are designed, developed, certified and manufactured for use in IS environments. Exact requirements can differ slightly country by country and range from the effects of static build-up on non-metallic castings and the assessment of cells through to the test parameters themselves.
A product certified as IS in Europe cannot necessarily be deemed to be intrinsically safe in the US or Canada, or vice versa. Ideally, a device should be certified to the local standard which defines the requirements for safety.
But what if the product you want doesn’t carry that local standard? What are the options?
To illustrate your choices we’ll use the example of taking photographs within a Class I Div 1 (B) hazardous (explosive) area in a US plant. In theory, it is possible to use a standard off-the-shelf digital camera, particularly if there is no certified option available. However this requires significant time and effort to raise hot-work permits, risk assessments and method statements. You’ll also need to use supplementary safety equipment such as a gas detector, as a minimum.
The other option is to use a camera that holds similar certification. For example, you could choose an ATEX certified camera that has been tested for use in explosive areas. Its tests have been performed to European standards but the obvious safety qualifications of this device provide far better protection than its off-the-shelf counterpart.
In this case, the plant safety officer would simply need to assess the camera, its certification and suitability for the task. However, this procedure is normally a one-off and far simpler than raising permits as in the first option.
Ultimately, using a device – in this case a digital camera – that is properly tested and certified for use in your hazardous (explosive) atmosphere is the correct method. This ensures that your facility is maximising the safety of its employees and is in compliance with local rules and regulations, backed up by third party certification and verification.
Explosion-proof – why use it?
So, why would anyone want to use explosion-proof (XP) as a protection concept? The answer is simple – power.
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 limitations can cause problems with devices that normally operate in a high power range. A camera strobe flash cannot be part of an IS device because it is powered by a capacitor which has a discharge greater than that accepted by the IS standard. An IS digital camera therefore has no strobe flash facility and, in low light situations, the imagery would be extremely poor. This can be overcome by using LED illumination. However the intensity of the LED can never provide the same level of brightness as a professional strobe flash. After all, how many professional photographers use an LED flash?
By using XP as a protection concept, ‘high’ power devices, such as a camera flash, can be used safely in an explosive area. An XP device is designed and tested to contain any explosion which may occur, and channel the expanding gas into the outside world via carefully calculated and constructed flame paths. As the gas travels through these flame paths, it cools preventing ignition of the external atmosphere, protecting the operator and the facility from harm.
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.
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