Connectors are dumb, passive devices with no power and no intelligence. Connectors convey power and signal between electrical units. Electrical engineering systems cannot work without connectors. Quality and reliability of connections are of paramount importance to the engineer. We can say without risk of being challenged that a loose connection is, probably, the worst enemy of the electrical engineer. It can cause disruption in operation. Intermittence in power connectors can go far beyond disruption. It can even cause sparking, localized heating, and a fire hazard. Therefore, connectors must be made to give reliable, uninterrupted connections. Locking mechanisms are particularly important where either vibration or relative motion between the connected units is present. Although there are many connector types which are just pushed in, but most connector parts have locking mechanisms designed into them.
Types of Locking in Connectors with Locking Device
A locking system is a mechanical device which aids the coupling and uncoupling of the mating parts and secures them in an optimal operating position. It helps to ensure and maintain the basic function of electrical continuity. Some connector designs also provide ingress protection, ie, protection against ingress of dust, vapor, etc. which could cause rusting of mating pins. Many locking devices include aids to proper alignment of mating pins. This feature is of particular importance in connectors with a large number of contacts. It is also important in power connectors for the reason described above. Clearly, there is a wide variety of locking mechanisms available in connectors. Some common ones are briefly discussed below.
Thread Screw Coupling:
This is the most secure connector locking mechanism. The pins on the male are visually aligned with females sockets before the locking shell is turned. Some connector systems may have mechanical key alignment systems which consist mating protrusions and recesses on the mating parts. Mating is possible only when the pins are properly aligned. A little manual push on the connector parts will allow threads of the lock mechanism to engage. Thereafter as the lock shell is turned, screw force moves the pins into the proper sockets. The screw should be turned till fully engaged.
The locking shell will provide mechanical strength against twist as well against ingress of dust, water, etc. The screws may have coarse pitch (Coarse thread screw coupling), or a fine pitch (Fine thread screw coupling), depending on the pin size and other mechanical considerations. Thread-locked connectors will, perforce be round. Round shaped conductors may have slight disadvantage in that they will not use panel space optimally.
Connectors Locked by Screws
Surely, connectors of non-round shape will need other mechanisms to hold them in place. One scheme is to engage the connectors and then lock them together in place through aligned holes on both parts. Multipin rectangular connectors are an example of candidates for such locking.
This is another method for locking non-round connectors. A clip on either the male or female body locks into recesses on the other thus holding the two parts in place. Again, the clips are applied after the pins have been fully engaged manually.
Push & Press to release coupling
Such a mechanism is simple and applicable to connectors of various shapes of cross-section. It allows a quick connect and quick disconnect at relatively low cost. The user aligns the pins and pushes the two parts together. As the mate a rectangular tooth on the body of one part engages flexible tooth on the other part. The tooth design is such that the connectors are firmly locked in place and cannot be withdrawn unless a tab on the flexible tooth pushes it out of the way. Press the tab and hold it there at the same time drawing the two connector parts apart. Examples of this type of locking are the various RJ connectors often used in telephone and internet services. This type of locking is also popular in medical equipment. By the way, what is meant by RJ? RJ stands for “Registered Jack”
Most of us have seen bayonets in face to face fights on the TV. How fast can the fighter lock and unlock his bayonet onto or from his rifle. The bayonet connection is best understood by soldiers. This is probably the simplest good locking device for electrical connectors. Just align, push and turn one part and it locks in place on the other, The hand will feel the locking motion, and the ears may hear a click (in case of larger connectors) as it locks in place. Since a turning is involved, the bayonet locking is suitable only for round connectors.
Further, it is meant for a single contact. Locking is implemented by placing two diagonally opposite radial pins on the inner body. A cut in the outer shell accepts the pins. As you push and rotate the connectors mate deeper and finally the pins enter a recess from which they cannot come out unless the connectors are intentionally pushed and rotated back.
Twist-lock connectors are simply pushed onto a locking male receptacle or plug and slightly twisted to lock the two in place. This will prevent the two parts from coming loose. They are used in many industrial applications where a secure connection is desired to prevent accidental disengagement in environments of vibration or relative movement. An example is the Bryant 15 Amp Industrial Grade Locking Connector, made to NEMA Configuration L7-15P.
Other Types of Connectors with Locking Device
Clearly, there are other types of locking devices in the market like push-pull locking, and breakaway locking. Moreover, the breakaway locking permits a quick and clean disconnect. Also, it is more popular non-electrical applications like fluid pipe coupling.
NEMA Identification for Connectors with Locking Device
Connectors made to NEMA (National Electrical Manufacturers Association) specifications have an identifying letter “L” in the nomenclature to state the presence of a locking device. Absence of the letter L at the start means it is not a locking connector. For example, the NEMA L7-15P, has a locking device which is indicated by the letter L before the number 7. The number 7 is specified for a 277 v rating and a ground connection. 15 is the Ampere rating and the P indicates it is a plug, not a socket. For a socket the last letter would be an ‘R’, indicating a receptacle.