Monday, February 25, 2008

There are now more variations of cable gland available than ever, and with more choice comes the requirement for more knowledge in order to make the right selection. Considerations such as suitability for purpose, sealing, safety and installation time all need to be balanced against cost, which is always an issue.
The need to know is universal, as is the responsibility for the selection; the decision can fall to the electrician, electrical fitter, distributor, specifier or designer, working freelance on behalf of a contractor or an OEM all need to make the right choices, for the right reasons.

It is as easy to overlook a cable gland, as it is to under-specify, or, to over-specify it. With a huge range of fast new push-fit designs, tighter regulations and new materials coming onto the market it is essential to look at what is fit-for-purpose. Essential criteria include the quality of the hole available for the cable entry, the thickness of the cable, the ingress protection required (IP rating), the mechanical cable retention strength required, and the environment it is being exposed to.

Exposure to heat, weathering and any fire requirements such as building regulations, halogen free requirements, Ex rating or ATEX zones have to be considered. One of the original and simplest types of cable gland is the 'stuffing gland', offering basic sealing and protection for cables passing through panels or bulkheads. A locking nut is used to apply pressure via a skid washer onto the sealing rings that spread to form a seal around the cable. This method of cable feed-through achieves three basic functions.

First, it protects the cable from possible damage due to any sharp edges on the bulkhead/wall/cabinet. Secondly, it provides a barrier to moisture and debris entering the equipment housing (IP ratings are generally low as they will not withstand pressurised water or immersion). And, thirdly, it provides a degree of cable retention. The 'stuffing gland' has been around for many years and is used in an enormous number of applications today. Typically, however, a major problem with these particular types of traditional gland is the limited mechanical performance and its environmental protection rating of IP54.

Any application that requires better cable retention - for example superior mechanical strength, a wider clamping range or a higher ingress rating of IP65 or IP68 - may need a dome topped cable gland. These glands are also more aesthetically pleasing, looking more modern and smartening up an installation. A number of variations of this type are available in both nylon and metal but the salient features that ensure superior performance to the traditional stuffing gland are in its construction.

The cable is again fed through a sealing ring but in this case, the sealing ring is placed within an 'iris' that closes onto the sealing ring, clamping the cable and forming a high IP seal as the dome top is screwed down. The German standard thread type is known as PG (DIN46255 to 46259 etc) and has been replaced by metric thread standards (EN50262) introduced back in the late 1980s. The PG standard was given a transition period of 10 years to phase out and EN50262 became fully applicable in March 2001 although PG glands are still around, and judging by current sales volumes will be around for some time yet.

Many different cable gland variations exist in dome top configurations covering a variety of general industrial applications. Specialist applications that require additional protection over and above the common plastic variations include: products designed to meet the requirement of ATEX 100a for hazardous areas such as petrochemical; and glands designed for EMC compatibility and certified by VDE in compliance with VG95373 part 40. Stainless-steel versions should be considered in food and chemical process applications where washdown is essential for hygiene.

Nylon and plated brass glands are best suited for general industrial or marine applications. Most new developments have focused on speeding up the process of cable sealing and retention, offering push-fit glands and grommets in a range of designs.
The main issue with these variations is that they provide limited mechanical grip and will only seal up to IP67. Because the retention material is soft, it is generally less resistant to extreme heat, but can be more resistant to corrosive atmospheres and liquids. They are not currently suitable for any Ex applications.

There are a number of membrane plug/grommet products available that offer good sealing up to IP67 that are very fast to use. As a grommet they are a push fit design both to install and to feed the cable through. Recent grommet designs will allow for a variety of cable/conductor widths. They are resistant to vibration and will provide for easy adjustment. Again not suitable for Ex applications or extreme heat and most don't provide a great deal of cable retention force, unlike a clamped dome topped gland.

There are a range of fast-fit grommet seals that clip into standard drilled and knockout holes that provide positive cable retention. The Klikseal from TST for example is moulded using two materials, a softer compound for the body/seal, and a stiffer inner material that applies positive clamping and cable retention. This design will accommodate a few millimetres of wall thickness, self adjusting up to 5mm, and a range of cable diameters for each size. The added benefit of these designs is that they do not protrude far from the surface, only 5mm, unlike an equivalent clamping cable gland at 35mm, being flush to the surface means they are ideal for applications where space is at a premium.

Many contractors and OEMs are focused on reducing inventory and reducing overall line items, which is where these products really score. No tools are needed to fit them once there is a suitable aperture available, and the manufacturers claim they deliver a 90% time saving, crucial when there are a high number of entries to be completed in an application.

Enclosure manufacturers have also developed junction boxes that are manufactured using dual materials, injection moulding a softer compound into the wall design so that it forms self sealing cable entries. These will generally take a range of cable sizes providing sealing up to IP65 for a cable that is simply pushed through the flexible membrane. More suited to indoor applications or instances where there is secondary protection available such as a control cabinet, these junction boxes can certainly speed-up installation and assembly, particularly when the enclosures arrive onsite prepopulated with terminals and breakers etc.

One of these products is a junction box capable of connecting two or three round-profile cables via an integral connector block that features a similar sealing method to the dome top glands. It is moulded in one piece and hinged so that the threaded sections of the cable entries fold together. The cables are then inserted through a seal and locking cap and the cap screwed into position to complete the assembly. In conclusion, the humble cable gland can be a boon when it is specified correctly, speeding-up production and installation time, and a liability if it is wrongly specified.

The consequences of a water leak into a junction box can be catastrophic, not just for the internal components, but more importantly for the process, building or machinery that relies on that connection. Particularly relevant in industrial ATEX environments where the supplier and the fitter can be held liable when something goes wrong. Suppliers such as Hylec-APL provide free impartial advice and product samples to help ensure the right connection is made every time. Terry Spriggs is Product Marketing Manager for Hylec Components, one of the UK's largest suppliers of cable management equipment.

Wednesday, February 06, 2008

The Fluke VR1710 voltage quality recorder is designed for single-phase power quality troubleshooting and preventative maintenance applications. Designed as an easy to use solution for detecting and recording power quality problems, the VR1710 is a single-phase plug-in voltage quality recorder that can help maintenance and facilities personnel in utilities, industrial environments and large service organisations to maintain the power quality essential to business operations.

The Fluke VR1710 can help users pinpoint the root causes of voltage problems by recording data specified by a user-selected average period on voltage trends, dropouts and power quality parameters, including RMS average, transients, flicker and harmonics up to the 32nd.

It features continuous recording of all values with no gaps, making testing more efficient by enabling users to gather data on both voltage and events with a single instrument. The recorder plugs directly into any single-phase mains power socket and includes a USB connection and Fluke PowerLog software for quick instrument setup using a PC and quick data downloads.

PowerLog software also includes graphical tools for data presentation that show RMS voltage and harmonic trends, actual transients, summary information and statistics in accordance with EN50160.

It measures RMS average, minimum and maximum values and checks whether the socket outlet is providing voltage within tolerance. It checks whether distorting loads are affecting other equipment. High levels of harmonic voltage may cause overheating in neutral conductors. It quantifies the affects of switching loads on lighting systems and can capture intermittent, momentary events that may be affecting equipment. The full waveform is captured with date, timestamp and duration.