The protection technique encapsulation (Ex ‘m’) derives its explosion-protection properties from the fact that the encapsulation excludes the explosive atmosphere from the electrical equipment, thus isolating the explosive atmosphere from any arcing, sparking or hot surfaces.
The protection technique Ex ‘m’ may be used in conjunction with other protection techniques, for example Ex ‘e’ to allow the connection of a power source or loads. In this situation the encapsulant forms part of the equipment insulation and therefore is subject to all the same requirements of insulation, namely for its voltage rating and comparative tracking index (CTI). The encapsulation also protects the electrical equipment from the ingress of moisture.
Traditionally, Ex ‘m’ explosion-protected electrical equipment has not been subject to the activities of repair and overhaul. In fact, overhaul has been restricted to cleaning and inspection of the outer-exposed surfaces of the encapsulant.
However, with the introduction of softer adherent silicone-based encapsulating compounds that maintain their insulation properties to the same degree as the harder epoxy-based compounds, it has become possible to design Ex ‘m’ explosion-protected electrical equipment that may be repaired by cutting or picking away the encapsulant to expose repairable components within. The replacement of a fuse is a typical example of the type of component that may be replaced in Ex ‘m’ protected equipment.
The equipment must be designed so that it is repairable and the repair activity must be restricted to the part(s) that have been designated as repairable by the manufacturer and recorded as such in the equipment’s certification/approval documentation.
9.2 REPAIR/OVERHAUL PROCEDURES 9.2.1 Removal of encapsulation material
The exact location, surface boundaries and depth of any excavation of encapsulation material should be determined from the manufacturer’s certification/approval documentation and the repair or overhaul procedure. Under no circumstances is exploratory removal of encapsulation material to be undertaken.
The method of removal of encapsulating material should be in accordance with the manufacturer’s certification/approval documentation and repair/overhaul procedure. Once the component to be replaced and its termination(s) have been exposed, the component can be removed.
The opening made in the original encapsulation material to gain access to the faulty component(s) should be made in such a way that it has—
(a) a greater cross-sectional area at the bottom of the excavation than the cross-sectional area at the opening in the original surface of the encapsulation; or
(b) an undercut around the opening in the original surface of the encapsulation not less than 10% of the greater axis of the opening.
9.2.2 Replacement of components
Only those components listed in the original manufacturer’s certification/approval documentation should be replaced. The faulty component should be replaced by an identical one.
Fuses should normally be replaced in any circumstance either with a replacement of identical type or, if this is not possible, with an alternative that has—
(a) the same rating or less;
(b) the same prospective current rating or more at the same or greater voltage;
(c) the same type of construction; and (d) the same physical size.
Printed circuit boards may be replaced by exchange units that have been obtained from the manufacturer. The printed circuit board should be terminated in the same manner as the original.
9.2.3 Preparation for replacing encapsulation
The inner surface of the excavation into the encapsulation material should be free of the following:
(a) Loose or partially attached encapsulation material that may remain after the excavation process.
(b) Oil, moisture or any other material that may interfere with the adherence of the replacement encapsulating material to the original encapsulation material.
If required by the encapsulation procedure, the surfaces of the excavation should be treated with a primer.
9.2.4 Encapsulating material
The replacement encapsulation material should be identical to the original encapsulation material. The use of any other type of encapsulation material other than that stated in the certification/approval documentation is considered a modification.
The replacement encapsulant should be mixed and prepared in accordance with the manufacturer’s instructions. It is important that the mixing process does not introduce air bubbles into the mixture.
The chemistry of encapsulant materials may vary between batches and it is good practice for service facilities using encapsulant materials to confirm the mixture and curing properties of each batch prior to use.
Records of batch numbers for all components of each mixture of encapsulant should be retained.
9.2.5 Pouring encapsulation material
The replacement encapsulant should be poured into the excavation so that—
(a) the surface of the replacement encapsulant is parallel to the surface of the original encapsulant in all planes;
(b) it does not entrap air or leave voids; and
(c) it fills the entire excavation to a level the same as, or higher than, that of the surrounding original surface.
9.2.6 Visual inspection
Each piece of Ex ‘m’ explosion-protected equipment should be subjected to a visual check.
No visible damage should be evident, such as cracks in the compound, crazing, exposure of encapsulated parts, flaking, shrinkage, swelling, decomposition, discolouration, failure in adhesion or change in hardness.
9.2.7 Reclamation
Because of the removal and addition of material to gain access to faulty components, reclamation is considered the same activity as repair for the Ex ‘m’ explosion-protection technique.
S E C T I O N 1 0 E X ‘ n ’ N O N - S P A R K I N G