Views: 0 Author: Site Editor Publish Time: 2026-03-24 Origin: Site
Unscreened separable connectors are one of those components that tend to get selected too quickly. An engineer sees a voltage rating that matches, a current rating that looks right, and a price that fits the budget, and a selection is done.
But that approach, while understandable given the pressures of project timelines, overlooks the variables that actually determine whether a connector will perform reliably over ten, twenty, or thirty years of service. The wrong choice might not cause an immediate failure. It might cause a slow, insidious one, or a compatibility issue that doesn't surface until commissioning day.
This guide walks through the factors that genuinely matter when specifying an unscreened separable connector, in a practical sequence that mirrors how an experienced engineer would approach the task.
Before diving into selection criteria, it's worth making sure we're all starting from the same place. Not everyone reading this article will have the same level of familiarity with separable connectors, and assumptions about baseline knowledge can lead to confusion further down the line.
An unscreened separable connector provides a demountable electrical connection in medium voltage (MV) systems — typically at voltage levels up to around 24 kV, though specific ratings vary by manufacturer and design. The "separable" part means the connector can be disconnected and reconnected without permanent modification.
The "unscreened" part means the connector does not incorporate a metallic screen (also called a shield) around the insulation body. This distinguishes it from screened separable connectors, which include a grounded metallic layer that controls the electric field distribution around the connector.
Common applications for unscreened separable connectors include connections to medium voltage switchgear, distribution transformers, ring main units (RMUs), and other distribution equipment where the ability to disconnect and reconnect cables quickly and safely is a priority. They're widely used in underground distribution networks, commercial and industrial power supplies, and renewable energy installations where medium voltage cable connections are needed at the point of equipment interface.
This is the core of the selection process. The factors outlined here aren't theoretical; they're the variables that, if overlooked, are most likely to cause problems in service. Addressing them systematically during specification is the most reliable way to avoid costly surprises later.
The rated voltage of an unscreened separable connector tells you the maximum system voltage at which the connector is designed to operate continuously. It's the first filter in any selection process; if the voltage rating doesn't match or exceed your system voltage, the connector is immediately unsuitable.
But voltage rating alone doesn't tell you everything you need to know. The rated voltage is typically expressed as Um (the maximum voltage for equipment), and it corresponds to specific requirements for power frequency withstand voltage and lightning impulse withstand voltage.
Current rating is the next critical parameter, but again, the headline number can be misleading if you don't look deeper.
The continuous current rating tells you the maximum current the connector can carry continuously without exceeding its thermal limits. But you also need to consider the short-circuit current rating. A connector that handles normal load current beautifully can fail catastrophically if it can't withstand the fault currents that flow during a short circuit.
Thermal performance under load is equally important. The contact interface between the connector and the bushing or equipment terminal generates heat due to contact resistance. If the connector is poorly designed, poorly installed, or made from materials that degrade over time, that contact resistance increases, generating more heat, which further degrades the contact.
The installation environment has a direct and significant impact on connector performance and longevity. An unscreened separable connector installed in a clean, dry, temperature-controlled indoor switchgear room faces a fundamentally different set of challenges than one installed in an outdoor, coastal, or underground pit environment.
Temperature range matters, both the extremes and the rate of change. Rapid temperature cycling can cause differential expansion between materials, potentially loosening contact interfaces or creating micro-cracks in insulation. Humidity and condensation can degrade insulation performance, particularly if moisture reaches the contact interface or penetrates the insulation material.
The mechanical aspects of connector selection are often underestimated. How many times will the connector need to be connected and disconnected over its service life? Some connectors are designed for frequent operation, while others are intended for infrequent disconnection, maybe only a handful of times over decades.
If you're specifying a connector for an application where regular disconnection is expected (for example, at a transformer that's periodically taken offline for maintenance or testing), you need a connector rated for that level of mechanical cycling.
The insulation body of an unscreened separable connector is its primary protection against electrical breakdown, and the material it's made from significantly affects performance.
Silicone rubber and EPDM (ethylene propylene diene monomer) are the two most common insulation materials used in these connectors. Silicone rubber offers excellent hydrophobicity as it causes water to bead up and roll off rather than forming a continuous film on the surface. It also maintains good flexibility across a wide temperature range and has strong resistance to UV degradation and ozone.
These properties make silicone rubber the preferred choice for demanding environments like outdoor installations, coastal areas, high-humidity locations, and applications with significant temperature variation.
EPDM is typically less expensive and performs well in clean, dry environments. However, it tends to lose its hydrophobic properties over time, making it more susceptible to tracking and surface degradation in contaminated or humid conditions. For indoor, controlled-environment applications, EPDM can be perfectly adequate. But for anything exposed to the elements or pollution, silicone rubber is the safer long-term choice.
Before you go too far down the path of specifying a particular unscreened connector, it's worth pausing to confirm that an unscreened design is actually the right category for your application. Getting this fundamental decision wrong renders all subsequent specification work moot.
The absence of a metallic screen means that the electric field around the connector is not actively controlled by a grounded conductor.
In a screened connector, the metallic shield provides a defined ground plane that shapes the electric field, ensuring it's distributed evenly and doesn't concentrate at points that could lead to partial discharge or breakdown. Without this screen, the insulation material and the connector's geometry alone must manage the electric field distribution. This is perfectly adequate at lower medium voltages where the electric field stresses are within the capability of well-designed insulation systems. But at higher voltages, the absence of a screen can lead to unacceptable electric field concentrations, making screened connectors necessary.
Unscreened separable connectors are generally appropriate for systems operating at rated voltages up to around 24 kV, depending on the specific design and manufacturer. They're commonly used in distribution networks at 6.6 kV, 11 kV, and in some cases up to 24 kV.
Their simpler construction makes them less expensive than screened alternatives, and their installation is typically more straightforward. However, if your system voltage exceeds the rated capability of available unscreened designs, or if your application involves conditions that create enhanced electrical stress, a screened connector may be the more appropriate and safer choice. Don't let cost savings drive you toward an unscreened connector if the application genuinely demands a screened one.
If there's one area where connector selection goes wrong most frequently, it's compatibility. The assumption that a connector will fit and function correctly with a given piece of equipment has caused more commissioning delays, rework costs, and in-service failures than almost any other specification error.
Interface standards exist precisely to prevent compatibility problems. Standards such as those defined by the IEC (International Electrotechnical Commission) specify the critical dimensions, tolerances, and performance requirements that connectors and their mating equipment must meet to ensure interoperability.
Even with interface standards in place, verifying compatibility with specific equipment is essential. Transformer bushings, switchgear cable entry ports, and RMU connections all have specific geometries, and not every connector fits every bushing.
Confirm the bushing dimensions, the insertion depth, the contact arrangement (whether it's a probe-and-socket or a tulip-contact design), and any mechanical locking features. If possible, obtain dimensional drawings or interface specifications from both the connector manufacturer and the equipment manufacturer, and cross-reference them. It takes time, but it's far less expensive than discovering a mismatch on site.
The connector doesn't exist in isolation; it terminates a cable, and the cable's characteristics directly affect which connectors can be used. Cable type (XLPE, EPR, paper-insulated lead-covered), conductor cross-section, insulation diameter, and the cable's outer sheath dimensions all determine whether a given connector can be correctly installed.
A connector designed for a 95 mm² XLPE cable won't necessarily work on a 95 mm² EPR cable if the insulation diameters differ. And a connector that's technically compatible on paper may not install correctly if the cable's bending radius or stiffness makes it difficult to position the connector properly within the available space.
Always check cable compatibility tables provided by the connector manufacturer, and if your specific cable isn't listed, contact the manufacturer directly for confirmation.
A connector that looks perfect on a datasheet can become a nightmare in the field if the practical realities of installation weren't considered during selection.
Medium voltage cable compartments in switchgear, transformer cable boxes, and underground connection pits are often tight. The connector needs to fit not just physically, but with enough clearance for installation tools, cable bending, and phase-to-phase spacing.
Proper installation of separable connectors typically requires specific tools; crimping tools for conductor connections, cable preparation tools, and sometimes torque wrenches for mechanical fittings. It also requires the right lubricants (usually silicone-based) to facilitate insertion without damaging the insulation surfaces.
Cleaning materials, abrasives for conductor preparation, and sometimes specialized test equipment are also part of the picture. These requirements should be confirmed at the selection stage, not discovered on installation day.
Selecting the right connector isn't just about getting it installed and energised — it's about ensuring it continues to perform reliably for the duration of its intended service life, with an acceptable level of maintenance and monitoring.
What's the intended service life of the installation? Is this a 30-year utility distribution asset, or a 10-year temporary supply for a construction project? The answer should influence your material choices, your quality expectations, and your willingness to pay a premium for a more durable design.
Can the connector be visually inspected in service without disconnecting it? Does it have provisions for diagnostic testing? How easy is it to disconnect and reconnect without degrading the contact surfaces or the insulation? These questions matter more than many specifiers realise.
Understanding how unscreened separable connectors typically fail helps you make better selection decisions.
The most common failure modes include contact degradation (increased resistance at the contact interface due to corrosion, fretting, or inadequate contact pressure), insulation tracking (carbonised paths forming on insulation surfaces due to contamination and moisture, eventually leading to flashover), moisture ingress (water penetrating the insulation system, reducing dielectric strength and potentially causing electrical trees or partial discharge), and mechanical loosening (the connector working loose from the bushing due to thermal cycling, vibration, or inadequate locking mechanisms).
The technical specification is only part of the picture. Who makes the connector, what standards it's been tested to, and what support comes with it are all factors that affect your risk.
For medium voltage separable connectors, the primary IEC standard is IEC 60137 for bushings and IEC standards covering cable accessories and connectors. Interface standards (such as those defining standard bushing dimensions and contact arrangements) are particularly important for ensuring interoperability. Beyond the product-specific standards, you should also consider relevant installation standards and testing standards.
Type tests are particularly important. A connector that has been type-tested to the relevant IEC standards by an independent testing laboratory provides a significantly higher level of confidence than one backed only by manufacturer self-certification.
Before finalising a connector selection, there are several questions worth putting to the supplier.
Can they provide type test reports from an accredited laboratory?
What routine tests are performed on production units?
Do they offer application engineering support to verify compatibility with your specific equipment and cable?
What's their track record; how many of this particular connector design are in service, for how long, and what's the field failure rate?
Can they provide reference installations in similar applications and environments?
A supplier who can answer these questions confidently and transparently is one you can work with.
Selecting the right unscreened separable connector is a process. It requires consideration of voltage and current ratings, environmental conditions, mechanical demands, material properties, compatibility with equipment and cables, installation practicalities, long-term reliability, and supplier credibility.
Each of these factors interacts with the others, and overlooking any one of them can compromise the performance, safety, or longevity of the connection.
