Electrical Cable Terminations from LML Products

Established since 1968, LML Products manufactures copper-tube terminals from 10-1000mm^2, along with many other high-current components, at its own facilities in Calne, Wiltshire, UK. All manufacturing is accredited to BS EN 9001. Parts are only manufactured from optimum-purity original mill-produced copper. In-house recycling is not used owing to the risk of trace contaminants that can seriously impair electrical and mechanical performance. Standard Terminals are manufactured from 99.9% pure Copper to BS EN 12449 / BS EN 13600 CW004A. Higher grades are available to special order.

• Heavy-Duty terminals are required for applications expected to survive high surge-currents. Design features can be specifically configured for their application (e.g. HV or buried connections etc.).
• Protected-Circuit terminals are intended to provide secure crimping and long life-cycles when working currents are actively limited to the cable’s normal rating.

The standard electroplated coating of tin maintains electrical performance for cables used with normal rated currents and with connection temperatures between -20°C and +80°C. A further range of coatings, including nickel and silver, is available to optimise reliability and performance in critical applications and extreme conditions. 

Both standard and bespoke tooling and dies are available for all our terminals along with detailed specifications of process parameters.

All our terminals have been extensively independently tested and are capable of compliance with EN 61238, BS 4579, RSE/STD/024-7A and many other performance standards, but please note that compliance defines a combination of cable, termination and crimping method and cannot be transferred to the terminal alone. Specifying this combination is a clear necessity to prevent a terminal defining connection quality if mis-applied to an inappropriate cable by an inadequate process. We are more than happy to provide further details on this subject and can also establish many more detailed aspects of electrical and thermal performance through testing, numerical analysis and computer modelling.

Certificates of compliance are also available for parts requiring verification of controlled manufacture or performance. Please contact us:

T: 01249 810000

E: sales@lmlproducts.co.uk

W: www.lmlproducts.co.uk

Appendix A: Coatings 

Coatings can protect copper from the surface oxidation which causes corrosion and degrades electrical properties, but great care must be taken at high temperatures as diffusion of many coating materials may cause serious damage to its bulk resistivity.  

For routine low-stress applications, tin protects copper contact surfaces with a low contact-resistance solder-compatible coating with MP c. 230degC. It is a perfectly adequate coating for electrical cables and their terminations up to about 100-150degC. Tin is also unsuitable for environments below circa -20degC; temperatures below this it can form an amorphous allotrope that can detach as a loose powder. LML’s standard terminals are normally supplied with a 5µ tin coating and are intended for use at controlled working temperatures between -10degC and +150degC. Nickel is a reasonable alternative to Tin for limited higher temperatures up to about 250degC; it does form a tougher surface oxide which can cause higher contact resistance than tin. Nickel should not normally be regarded as a solder-compatible coating owing to the resilience of its oxide surface.

At the atomic level, thermal vibrations of the copper structure can enable coatings to diffuse into the copper substrate at a rate that increases with temperature. Small amounts of tin or nickel both cause serious damage to copper’s electrical resistance if incorporated into the copper lattice by this process. 1% of tin increases copper’s resistance by 150%, will increase thermal losses by a factor of 2.5 and have similar impact on a cable’s temperature above ambient under working loads. The effect of nickel is very similar. Tin-coated fine strands in flexible cables can easily become permanently degraded to a dangerously resistive condition if allowed to overheat and core temperatures may be much higher than indicated by their surface. It is advisable to keep the current density and working temperature of such cables as low as possible but please contact us for more detailed information on this important issue. 

Silver retains stable surface contact resistance under most conditions and provides by far the best protective coating for electrical applications of copper. Its conductivity is the best available, slightly better than pure copper and, alloyed by diffusion or otherwise, silver has no detrimental effect on the bulk conductivity of a copper component or cable at any temperature; copper-silver brazing provides excellent high integrity connections. The benign resistive impact of silver’s surface oxides also make it an ideal metal for stable electrical contact. Silver coating is used for high-reliability contacts and bus-bars plus instrument-cables and their terminations.

Appendix B: Current Ratings

The current carrying capacity of an electrical termination is dependent upon the complete connection assembly, its acceptable temperature and its working environment.  

For a cable-termination, the main determinant of current rating is the cable itself, and a properly secured termination will not normally compromise this; sound cable terminations support the current loading expected by the cable and its power connections. Current rating is defined by temperature; an ideal current rating for any component and environment maintains operating temperature comfortably within defined limits. The high thermal conductivity of copper disperses heat from any local increase in resistance back into the cable itself at continuous working currents. This intrinsic heat-sinking is very effective in preventing localised high temperatures at connections and is a critical determinant of current rating. The same situation applies to the termination’s fastener-connection to a power source or load; it is important to determine whether this will act as a heat-source or heat-sink at the loading of interest. It remains however paramount to ensure that terminations are properly crimped, or soldered, onto the cable and that the termination is properly secured onto its connection with adequate area of contact; the area clamped under fastener washers should at least equal the cable’s cross-section. Protected-Circuit cable terminations are intended to carry the working current-levels and overloads rated for the cable in a system with circuit-breaker protection and heavy-duty terminations are physically more secure and intended to survive more profound surge-currents. LML also manufacture specialised terminations and terminated cables which are intended to provide safety-earthing or other extreme applications.

Properly specified wiring systems include circuit breakers which arrest any overload current before damage occurs to downstream cables. Unprotected surge currents have a very different behaviour to stabilised working currents however. With very high current densities, increases in resistance with temperature can cause heating effects to outstrip both external cooling and internal conduction to destabilise both temporal and axial temperature variations. Destabilised conductors can then escalate rapidly to destruction and high-resistance areas are no longer protected by thermal dispersion into the surrounding conductor. Local increases in heating are no longer dispersed but exacerbated and, without over-current protection, degenerate into failure-points.

These effects are generally well understood and can be effectively predicted by numerical models and mathematical analysis. LML can provide high-specification terminations which are designed to minimize the risks from such effects. The best proof of performance is provided by field trials and testing however and LML can provide specialised facilities for high-current injection testing either on-site or under laboratory conditions to check for sensitive areas in power circuits.

Appendix C: Working Voltage

Uninsulated Electrical Cable Terminations are current-carrying components and, as such, do not have intrinsic voltage limits.

Terminals are supplied in a form that is suitable for most applications up to c. 10kV.

Terminals used in higher voltage circuits may have particular features designed to assist installation requirements and practices in such applications:

• Outer edges may be smoothed with an applied radius and barrel flare may be replaced with an internal chamfer. These features are intended to reduce concentrations of electric field at external surfaces.

• Inspection holes may be omitted in order to provide a complete seal at the cable termination; this can be taken further by using solid or brazed-palm terminals. The intention here is to protect the cable itself from corrosion by limiting or excluding any ingress of water or air.

• Other specific design features may be incorporated in order to enable the terminals to properly connect a cable to its recipient.

If you have any particular requirements or queries please contact us directly.

Appendix D: Application and Installation by Crimping

Secure connection is a critical responsibility for any cable installer. In the first instance the selected cable must be fit for purpose and correctly rated for the intended duty under the expected environment with termination properly specified to match the cable. It is also important that the termination fastener-connection will be properly secured to its recipient; the fastener-hole must be properly specified. The area of contact is determined by the area of compression underneath the fastener (normally the area of its washer) and, to match the cable’s working-load, this should be approximately equal to the area of the cable cross-section. Check carefully if there is any doubt on any of these important details.

There are many forms of crimp compression, but stable connection-integrity with low contact-resistance between termination and cable strands demands mechanical-integrity in the crimped connection. This can only be achieved through correct preparation, process and design; the prerequisites for sound, stable connections. 

Preparation for a crimp with high mechanical-integrity demands the correct termination design and metal cross-section for both cable and application. Sound connection requires adequate crimp length along the cable axis and substantial metal distortion during the formation process to bring all cable-strands into close contact and work-harden the receptacle. Stability in the connection requires the crimp-hardened receptacle to form a satisfactory 3-dimensional structure. Multiple narrow crimps, axially-spaced along the receptacle, may assist compression and stability. 

Arguably, the most stable crimp-form is purely circular and can be produced under factory conditions by rotary swaging. In the field, a hexagonal crimp is a reasonable compromise. Care should be taken however as initial compression accumulates structural rigidity in the outer cable strands. This is equivalent to the strength obtained from compression in semi-circular stone or brick arches and may restrict core cable-compression. Central strands then suffer poor connection and poor hermetic integrity guarding back-penetration of moisture into the cable if total metal distortion is inadequate.

Indent crimping does overcome the geometric restraints of cylindrical compression and derives strength from the final 3-dimensional form. The indent can be singular, opposed against a semi-circular cusp, or an axially-symmetric set (usually of 3) around the cable termination. Properly applied with adequate pressure, either arrangement provides sound connection and low contact-resistance.

There is no single definitive and optimum crimp, the best proof of integrity comes from field experience and/or testing (see page1). We are always pleased to advise however and can offer multi-kA injection-testing and micro-volt connection-testing services both on-site and in-house.