TYPES OF TERMINATIONS AND CONNECTIONS FOR INSTRUMENTATION

There are many different ways of joining or terminating conductors. The different methods of termination are divided into two groups:

• Heated terminations (e.g. brazing, soldering)
• Mechanical terminations (clamping, bolting, etc.)

This paragraph will cover mechanical terminations only.

Mechanical terminations

The advantage of mechanical terminations is that they are strong but the connection is not permanent. It can therefore be taken apart easily for repairs or changes to the circuit. The disadvantages are that the terminals can oxidise and screwed joints can become loose over a period of time (This is why you need regular maintenance operations to retighten the connections).

The most common types of mechanical terminations are:

• Pillar Terminals
• Screw Terminals
• Nut and Bolt Terminals
• Strip Connectors
• Claw washers
• Split bolt connectors

Pillar terminals

You often see pillar terminals in the plugs on household appliances.

A pillar terminal has a hole through the side where you insert the conductor. A set screw is tightened onto the conductor. If the conductor is small in relation to the hole, it should be doubled back. You can put two or more conductors in the same terminal by twisting them together.

You must be careful not to damage the conductor by tightening the set screw too much.

Screw terminals and nut-bolt terminals

When fastening conductors under screw heads or nuts the conductor should be formed into a loop. You can do this easily using round-nosed pliers.

The loop should be placed so that when you tighten the screw or nut you do not cause the loop to open.

Claw washers (also know as toothed washers)

You can use a claw washer to prevent the loop from opening when you tighten the screw.

Place the plain washer on top of the loop. Then place both the loop and the plain washer on the claw washer. Tighten until the metal teeth of the claw washer are flat on the terminal loop.

Strip connectors

Strip connectors are sometimes called terminal blocks. Strip connectors are a group of brass connectors fitted in a line in a moulded insulated block.

The conductors are held in place with a grub screw in the same way that a set screw grips the conductor on a pillar terminal. The conductors should be pushed well into the connector.

This prevents the grub screw only gripping the end of the conductor. It is important to get a good, clean, tight connection. This will prevent high resistance contacts which would cause the connection to overheat. It will also prevent loose contacts which might come apart.

Some connectors have pressure plates which produce better connections. A pressure plate spreads the pressure over the connection and provides a better contact between the two conductors. This is especially important when you are dealing with high current circuits. They are also good for multistrand conductor terminations.

Grub screw or pressure plate terminals are normally used for terminations in electrical equipment, e.g. switchgear, starters, breakers, contactors, etc.

Split-bolt connectors

Split-bolt connectors are used to join two or more cable ends together. They are also called line tap connectors. This is because they are often used to tap off a conductor (line) without switching off the circuit.

Crimped connections

Crimping is a quick and effective way of joining different types of termination devices to cable conductors. Do not crimp a rigid core cable since it can be directly connected to the terminal block without any problems; it is even recommended never to use lugs on rigid cores! But when dealing with stranded cores or even flexible wires it is a professional error not to use crimped terminations when the receiver’s connectors are not adapted to the wires.

Crimped lugs

Crimping requires the use of a special tool (crimping tool) to apply enough pressure to form a good connection between the crimp connector and the conductor. A correctly crimped connection has high mechanical strength and good electrical conductivity.

Crimped connections are quicker to make and cheaper than soldered connections. Crimped connections are often used for small cables and made by a hand-operated crimping tool. But before crimping, you need to choose a lug or a sleeve and these depend on:

1. Wire material: copper or aluminium – It can be also bimetal, crimped on aluminium cable and connected to a copper-based bolt/seat.
2. Wire size: from 0.5 to 600 mm²
3. Type of connection: pin, fork, blade, spade, ring, etc.

With so many different sizes and types you are obviously not going to be able to use the same crimping method or the same crimping tool for all of them!

Crimping tools

Hand crimping tools often have jaws which can be changed to fit the different shapes and sizes of crimp connectors. You crimp the connection by squeezing the handles together. The handles cannot be released until full pressure is applied to the connection (except for the “fastener” type crimping tool).

Then the ratchet releases the handles. This ensures that the correct pressure is applied to the crimp connection. A power-operated crimping tool (hydraulic) is used for crimping the large conductors of high current cables.

With this type of tool for making a compression crimp, the correct die must be used (depends on lug/sleeve diameter). The hydraulic crimping tool can be hand-operated, battery-operated or have a separate pump which can also be manually, pneumatically or electrically operated.

Making a crimped connection

1. Strip the insulation from the end of the stranded conductor.

This systematically applies for stranded and flexible cable

2. Twist the strands together with pliers so they all fit into the body of the crimp connector.

3. Place the barrel of the crimp connector over the conductor

4. Place the barrel of the crimp connector in the crimping tool jaws.

5. Crimp the connection by squeezing the handles of the crimping tool, or start to pump

6. Squeeze the handles together until the ratchet releases them (the die is completely closed or the punch is at its maximum with the hydraulic unit).

Open the handles and remove the tool from the crimped connection.

7. Check that the connections are well-formed and perfectly made.

The wire has been stripped to the correct length corresponding to portion ‘B’. The insulation is well inside its barrel. The finished crimp is firmly secured.

Use the correct crimping method

With a hand crimping tool for small lugs, it is a simple operation. Once you have to use hydraulic equipment, it means that you have to connect cables of large cross-sectional areas and you must:

Decide on the compression method and dies to be used.

1. Select the correct shape of die for the job.
2. Select the size of die for the diameter of the lug.
3. Ensure that the correct hydraulic pressure is used (check the manufacturer’s recommendations).

Decide on the punching method to be used (bottom die)

• Slide the lug bed and punch the cover over a certain diameter only.
• Change the bed and punch according to the lug diameter if your tool can handle it.
• Check the manufacturer’s recommendations for the number of punches on the selected lug.

Position the lug correctly

This lug position is (of course) valid for all types of lugs and sleeves. With a preinsulated compression lug, do not forget to insert the wire’s insulation into the part of the lug designed for this purpose.

Crimp the cable lug or connector using the correct tool, take account of the crimping direction (when several crimps are required).

Discard any “failed” crimps

This kind of problem (lug cracking) can occur either due to a material failure (manufacturer’s responsibility) or to a wrongly used crimping tool (wrong die, excessive pressure).

Make at least a double crimp (large lugs)

The recommended method is to double crimp although single crimps may be suitable for smaller cables. The crimps should be at 90° to each other and positioned centrally within the contact zone to be crimped, as shown in the figure (in this case: hexagonal die).

If a single crimp is used, the crimp should be positioned centrally within the 35 mm zone shown in the figure. Crimp in the order shown to ensure a satisfactory crimp is obtained. Consult the workshop manual/manufacturer’s manual if any additional information or advice is required.