THE ADVANTAGES OF BUTT CONTACTS

Transcription

1 THE ADVANTAGES OF BUTT CONTACTS GENESIS Further to a corporal injury witnessed in the 50 s by Mr. Gilles Maréchal while a defective motor was plugged-in, he had the idea to rethink completely how these products were designed, to eliminate their potential danger. Keeping in mind the growing need for safety and reliability as well as the increasing voltages and intensities, he drafted specifications based on actual need, ignoring all the solutions that had been developed so far. These specifications were completed over time to match constantly the actual needs and new requirements of industrial users : In terms of safety : Ability to make and break full load currents and overload currents, Resistance to short-circuits, No access to live parts, IP seal achieved automatically, as soon as the plug is fully inserted into its socket, In terms of convenience : Ability to discriminate between different voltages with common parts, Ability to discriminate between different circuits under the same voltage, with common parts, Possibility to build up any required form of plug, inlet or socket from basic components, Possibility to mate plugs with different pin configuration and voltages with a single socket, provided they are electrically compatible. Possibility to replace any damaged part. The Decontactor tm using butt, spring-loaded, silver-nickel contact was born. The first silver-nickel butt contact plug and socket was launched in 1953, and this design is now more and more widespread worldwide. In 1996, all major car manufacturers assessed the butt contact as the most suitable design for electric vehicle conductive recharging. For this new application, the butt contact is already the standard of tomorrow. DECONTACTOR CONTACT PRINCIPLE Maréchal products use braided butt contacts in Silver-Nickel and a rated helical spring. Contact pressure is generated by the compression of this spring. This design is simple and allows wide manufacturing tolerances. Any plug inserted into any socket guarantees a minimum compression of the springs, that generates a minimum and controlled applied force. MTG

2 Springs, similar to those used on car valves, operate at only a fraction of their maximum rated resilience and can perform 100s of thousands of cycles without losing their properties. Car valve springs usually perform over 500 million cycles at a much higher rate, over decades. The flexibility of the braid ensures that the socket contact tip always mate perfectly with its counterpart on the plug side. Conversely to other designs, the electrical connection of Decontactors is made by means of a flexible metallic braid which links the fixed and the mobile parts of the female contact. This braid performs a static connection between the two parts, and its mechanical deformation allows the mobility of the moving part, without altering the path followed by the current. The hexagonal crimping performed at both ends of the braid ensures gas-tight connections, equivalent to a solid material and is unaffected by any possible corrosion or accumulation of foreign bodies. DISBREAKER CONTACT PRINCIPLE Opening contacts under load immediately produces an electric arc. At first, diminishing contact pressure melts the constriction area and creates an ephemeral liquid bridge. Rupture and vaporisation of this bridge ionises the intercontact area : an arc is immediately created within the very small distance between them, with an average voltage ca. 15 V that does not depend on the contact material. This initial voltage is small, compared to the operating voltage, usually 400 V. In this situation, the current path is practically not modified by the arc and its cancellation occurs only when the AC voltage drops to its natural zero, under provision that its re-striking is avoided. If the arc remains stable between the two contacts (stagnation time), its voltage increases only slightly with the distance. In Decontactors, the strong post-arc dielectric strength of Silver-nickel (which facilitates its instability and increases its voltage) and the length of the arc chambers, cater for this provision and prevent re-striking. For higher currents, or when significant overloads are involved, provision has to be made for a magnetic blow of the arc, to increase significantly its voltage. As the arc voltage increases, the current value is artificially reduced, in anticipation of its natural passage to zero. Disbreakers are fitted with such an arc blowing system by means of magnetic V- shaped blades that destabilises its cathode point and splits it across the blades. Disbreakers have a HP switch rating and can regularly make and break overloads ca. 10 time their nominal current. 14 MTG 2002

3 Plug and socket contacts are both made of a pin fitted with a solid silvernickel tip. When the plug is inserted into the socket, each plug phase contact touches the intermediate part first. While pushing the plug further, the mobile part moves around the stop and bends the spring until it is about to slip from the stop. Under spring pressure, the intermediate part escapes from the stop and rocks to connect with the live socket contact. This is the independent making, independent from the manual action of the operator. In the on (I) position, the plug is held by a socket mounted retaining ring. Turning this ring to the (0) mark releases the plug, which moves back. The spring-loaded mobile part abruptly slips from the socket live contact, and draws the arc across the blowing blades. Arc extinguishes. This is the independent opening, independent from the manual action of the operator. Disbreaker contacts have a cathode point (where the arc originates) which is different from the point where the load is carried, in the closed position. This allows performing numerous overload breaking operations without deteriorating the electrical continuity. LARGE TOLERANCES ALLOWED With butt contacts, the variables are the distances between the interior mouldings and the tips of the contacts : These variables are easy to check and maintain within precise limits, A decrease in contact compression makes almost no difference in the contact resistance, due to the linear response of the springs. This design provides users with a great confidence in product performance over time, despite their very long life expectancy, and all possible wear and tear that may occur. MTG

4 A patented design allows an even compression of all phases and neutral contacts Some play is necessary between the two cylinders of the plug and of the socket, to allow insertion. It also allows the plug to take an angle inside the socket, due to the single side catch (see quick disconnect). To overcome this lack of alignment, patented bossings are located on the plug outer cylinder and the socket inner cylinder. They compensate for the existing play between the two cylinders and ensures their perfect alignment. This allows an even compression of all the contacts and of the IP seal, that achieves automatically a rating of IP67 (DSN / DXN series). CONTACT MATING SEQUENCE To provide an optimum contact operation, Decontactor contacts mate in sequence : The central Earth (Ground) closes first, Then the Neutral, Then the Phases, Then the auxiliary contacts, if any. These auxiliary contacts can, therefore, be used as pilot contacts. On opening, the sequence is reversed: The auxiliary contacts, if any, open first, Then the Phases, Then the Neutral, Then the Earth (Ground). To ease stock holding and maintenance, all main contacts (Earth, Neutral and Phase) are generally identical. To perform this sequence, they are set in the moulding, plug side, at different levels. Socket side, all contacts rest at the same level. SELF-CLEANING Most of Maréchal products are fitted with a self-cleaning system of the contact tips, which allows the removal of any deposit on the contact surfaces. When the two contacts mate, they are slightly offset. During the compression of the spring, a special groove in the socket casing forces the plug to rotate, until the two contacts line up. This self-cleaning guarantees on-going performances, even in dusty or polluted environments. 16 MTG 2002

5 SAFETY SHUTTER Most sockets have a locked safety shutter which shields the live contacts when the plug is out. This keeps them clean and out of reach (to a 1mm wire), even if the cover stays opened. Only an electrically compatible plug can unlock the safety shutter. EASE OF OPERATION With the pin and sleeve design, the contact force is at right angle to the insertion / withdrawal force : a design that has three major drawbacks : The contact pressure must be sufficient to prevent excessive temperature rise, but is then limited by the need to keep the insertion motion easy. As the current rating increases, this task becomes more and more difficult to achieve. The necessary friction wears out the contacts, and diminishes contact pressure over time, Manufacturing tolerances allow wide variations in performance, even with new products. With butt contacts, the force applied to contacts is in-line with the insertion motion. Inserting a plug into its socket requires only a known and limited effort, and no effort at all to withdraw it. Contact wear is negligible and there is no risk of over-compression of the contact springs. QUICK DISCONNECT To provide for a quick load break in case of emergency, Decontactors are fitted with a single spring loaded latch button. The socket must be mounted in such a way that this button remains easily accessible. Upon depressing this button, plug and socket contacts separate in individual arc chambers, by the required distance, to perform isolation. MTG

6 On request, this recessed button can be replaced by a protruding emergency stop style button, which eases: Visual identification, Emergency operation, Operation with gloved hands. QUICK CONNECTION Connection is achieved in four steps : 1. Opening the lid 2. Introduction of the plug 3. Rotation of the plug (rest position) 4. Connection (latching) On most of products, two red dots located on the plug body and the socket body ease the mating of the two parts. 18 MTG 2002

7 SPRING-ASSISTED TERMINALS One of the main causes for failure of plugs and sockets is the loosening of their terminal screws : Trailing plugs and connectors are often roughly handled, Appliance or panel mounted products can be submitted to vibrations, Intermittent passage of current generates thermal cycles, Flexible conductors are made of multiple thin strands, likely to settle into place after tightening, Copper is a soft material with a low resilience, which easily yields under moderate pressure, ca. 15Kg /mm 2, a pressure easily generated by a simple screw. All these factors contribute to the loosening of the terminal screws. A simple terminal screw will not overcome all these potential problems : the conductor will loosen, the resistance will increase and the temperature will rise. And if one screw can loosen, two screws can loosen as well. A suitable design then needs to take into account all these parameters and provide for a constant applied force to the conductor. Maréchal products have a terminal design that compensates for strand settlement and copper yield. A constant force is applied to the conductor by means of an elliptical deforming ring, around a split terminal body. To prevent damage to the strands, the tightening screw has a soft head and its diameter is as large as possible. The conductor tunnel is offset to maximise the number of threads engaged by the screw. Under pressure from the screw, the split terminal body opens, until the ring holds it. The ring deforms elliptically in such a way that the combined resilience of the split body and of the ring compensates for any yield or strand settlement. At the same time, during thermal cycling, any dimensional variation due to the different expansion ratio of the various materials involved will be overcome. This elastic tightening also eliminates the risk of loosening under vibrations. Although re-useable numerous times, this design provides therefore a permanent connection that does not require to be checked regularly. We call it Tighten and forget. See Size and capacity of terminals. MTG