Making Fastening Joints - Course: Basic Skills and Knowledge of Electrical Engineering. Trainees' Handbook of Lessons
 3. Making Detachable Electrical Joints (Connectors)
 (introduction...) 3.1. Direct Connection 3.2. Indirect Connection 3.3. Plug-type Joints (Plug-and-socket Connectors)

3.1. Direct Connection

Direct connection is done in two ways:

Connection of the conductor by means of a wire lug to be bent or straight-line connection. Flexible conductors must be tin-coated before they are connected.

Connection of wire lugs

The following rules for connection are to be observed:

- A maximum of two aluminium conductors differing in up to two cross-sectional steps can be connected.

- A maximum of three copper conductors differing in up to two cross-sectional steps can be connected.

- Several conductors can be connected to connection bolts if a pressure piece is inserted after every two aluminium conductors or after every three copper conductors.

- A washer is to be inserted between the individual conductors.

- For connection of protective conductors or neutral conductors one conductor per connection point is admissible only.

- Each connection point is to be secured by a lock washer.

Why must no more than two aluminium conductors be connected to a connection bolt?
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Sequence of operations for making wire lugs

Wire lugs are bent on copper or aluminium conductors of up to 16 mm2 cross section. For flexible copper conductors the maximum cross section is 4 mm2.

Stripping of the conductor

The stripping length of the conductor depends on the diameter of the screw joint used.

It can be calculated to:

Figure 8 - Representation of a wire lug - 1 length to be stripped, 2 lug diameter, 3 distance from beginning of lug to conductor insolation

1 = (d + 0.5) + a
1 = length to be stripped in mm
p = 3.14
d = bolt diameter in mm
a = distance from beginning of lug to conductor insulation
(d + 0.5) = lug diameter

What tools are used for stripping of conductors?
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Bending of the wire lug

The wire lug is bent by means of a round nose plier in the direction of rotation of the fastening screw.

Figure 9 - Bending of a wire lug - 1 applying the round nose plier, 2 closing the wire lug, 3 bending of the lug to the centre of the conductor

The wire end is slightly pre-bent with the round nose plier. The conical plier noses are to be applied so as to give the correct lug diameter required. The round nose plier is then to be re-applied for finish-bending of the lug, preferably in one pass.

The lug should be tested on the screw joint to be used. If the lug does not have the required diameter, the plier noses are to be applied at a point where their diameter is slightly bigger to re-bend the lug. If the required diameter is reached, the lug is bent to the centre of the conductor.

Why must the wire lug be bigger than the connection bolt?
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Sequence of operations for connecting wire lugs by means of screw connectors

Figure 10 - Construction of a screw connector - 1,2 conductor, 3,4 washer, 5 lock washer, 6 screw

Unscrewing of the fastening screw

What tool is used for unscrewing a slotted screw and a hexagon-head screw?
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Composing the screw assembly

The screw assembly is to be composed in the following order:

- lock washer
- washer
- wire lug conductor 1
- washer
- wire lug conductor 2

The lugs of the conductors are to be inserted so that the bending direction of the wire lug complies with the direction of rotation of the screw.

Tightening of the fastening screw

The complete fastening screw is screwed into the thread by hand and then tightened with the respective tool.

What is the sequence of operations for making a bolt joint with 4 aluminium conductors?
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Figure 11 - Construction of a bolt joint with four aluminium conductors - 1 washer, 2 conductor, 3 lock washer, 4 nut, 5 bolt, 6 pressure piece

Straight-line connection

If lateral escaping of the conductor is limited by the screw head, bending of lugs is not necessary. The conductor is prevented from escaping by clamping saddles, distributor plates and connector blocks.

For straight-line connection attention is to be paid to the following:

- If conductors are fixed at the point of connection by means of clamping saddles, one conductor per saddle side must be connected only. The conductors may differ by one cross-sectional step.

Figure 12 - Clamping saddle connections - 1 clamping saddle

- If conductors are fixed by means of cap screw and distributor plate, one conductor per distributor plate must be connected only. The conductors must have the same cross section. The conductor(s) is (are) to be inserted so that it (they) will be drawn into the point of clamping when the screw is tightened.

Figure 13 - Connection of conductors by means of cap screw and distributor plate - 1 distributor plate, 2 washer

- If conductors are fixed by means of connector blocks, no more than two conductors, which have been soldered first, must be connected at each side.

Figure 14 - Connection by means of connector block

Sequence of operations for straight-line connection

The sequence of operation for connection by clamping saddles, distributor plates and connector blocks is the same:

Stripping of the conductor

The stripping length depends on the size of the clamped joint.

To avoid clamping of any conductor insulation, the stripping length must be approx. 1... 2 mm longer than the clamped joint.

Figure 15 - Determination of the stripping length - 1 size of the clamped joint, 2 1... 2 mm in excess

Unscrewing of the fastening screws

The fastening screws are unscrewed just as much as necessary to feed in the conductor.

Feeding in of the conductor into the clamped joint

The conductor is fed in under available washer or clamping saddles.

Why must the wire be applied at the left-hand side of a distributor plate?
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Tightening of the fastening screw

The fastening screw is firmly tightened by a screwdriver suitable for the size of the screw.