Effect of the cable capacitance

With AC controls with long control lines, low coil power ratings of the contactors and high control voltage, depending on the topography of the circuit, the capacitance of the control line can be in parallel to the controlling contact and practically bypass it when it is open. This can mean that when the control contact has opened sufficient current continues to flow via the cable capaci-tance causing the contactor not to drop out. An example may be a contactor that is controlled by a distantly located sensor (for example limit-switch).

Fig. 5.3-6

When the control contact switches off the cable to the contactor, the capacitance of the line causes at most a slight drop-off delay.

Fig. 5.3-7

If the long control line to the contactor stays live when the control contact is open, the current via the cable capacitance can prevent the contactor from dropping out. With pulse contact control, the capaci-tance of the lines acts twice, whereby the permissible line length is halved.

A worked example would be I_H = 0.25 I_CN U_H = 0.6 U_C cos φ = 0.3

I_H Holding current of the contactor I_CN Rated current the contactor coil U_H Drop-out voltage of the contactor U_C Control voltage

cos φ Power factor of the contactor coil (on-state)

The permissible cable capacitance is calculated at 50 Hz approximately to be C_Z ≈ 500 · S_H/U_C² [μF]

CZ Permissible cable capacitance [μF]

SH Holding power at UC [VA]

UC Control voltage [V]

At a typical cable capacitance of 0.3 μF/km the permissible line length for maintained contact control is

2 3

3 . 0

10 500

c H

z U

l = ⋅ ⋅S [m]

LVSAM-WP001A-EN-P - April 2009

5-8

With momentary contact control the line length is halved. Graphic presentation for the control voltages 110 V and 230 V see Fig. 5.3-8.

As the cable capacitance is very much dependent on the type of cable, it is recommended in case of doubt to obtain the specific value from the manufacturer or to measure it.

10 100 1000

1 10 S [VA] 100

l [m]

110V 230V

Fig. 5.3-8

Permissible line length in accordance with the above conditions for maintained contact control at control voltages of 110 V and 230 V at 50 Hz

l Line length

S Apparent power (holding power) of the contactor

If there are problems with respect to the permissible line length because of the line capacitance the following measures are possible in accordance with above discussion:

ƒ Application of an additional load (resistor parallel to contactor coil)

ƒ Use of a larger contactor with bigger holding power

ƒ Use of a lower control voltage

ƒ Use of direct voltage 5.3.5 Contact reliability

Electronic devices and circuits as commonly used in industrial applications, for example in PLC control devices and safety relays put high demands on the functional reliability of the controlling contacts, whether auxiliary switches of power switchgear or for example contacts of control units, sensors, function relays etc. The voltage to be switched is usually 24 V or even lower and the switching currents remain in the low mA range. Contacts connected in series (for example to safety relays) are frequently de-energized when they close and open, so that a switching

operation under electrical load never takes place.

0,5 mA 2 mA U L min. 15 mA

Operational range of PLC inputs in accordance with IEC 61131-2 (Programmable controllers – Part 2:

Equipment requirements and tests) and IEC 60947-1 Annex S (Digital inputs and outputs) for contact-inputs (digital input type 1) at a rated control voltage of 24 V. The contact load for ON can be between 30 and 15 V and 15 and 2 mA.

Copyright © IEC, Geneva, Switzerland. www.iec.ch

While at switching higher voltages and loads, a cleaning process by the arc takes place with every switching operation, with small signals special measures are required to ensure a high quality of contact making, that is to guarantee a high degree of contact reliability. At a typical PLC input resistance of several kΩ it is not a matter of mΩ as in power contacts. Good contact making can be for example prevented by

ƒ Films on the contact surfaces, originating from reactions with ambient gases (for example oxidation, formation of sulphide layers) or from deposits of volatile components of the ambi-ent atmosphere (e.g. that originate from production processes at the location or effluvia from plastics in the switchboard cabinet). Such films can usually only be identified with special devices and the cause is often hard to determine and eliminate.

ƒ Films on the contact surface that are caused by migration of metal material from the contact base and often interact with the first point and the switching operation.

ƒ Contamination of the contact surface that can emanate from the environment (open switch-ing cabinet doors durswitch-ing commissionswitch-ing!), from the interior of the switchswitch-ing cabinet or from the device itself. A problem that should not be neglected is the generation of foreign particles by the operation of the devices itself, for example due to abrasion.

Provisions for ensuring good contact reliability include

ƒ Selection of suitable contact materials (basic material and possibly surface coatings such as gold)

ƒ Avoidance of internal sources (for example materials and/or abrasion) that could have an adverse effect on contact reliability.

ƒ Use of high contact pressures that are able to break through tarnishing films, e.g. by appropriate shape of the contact surface.

ƒ Relative movement of the contact surfaces during circuit making that break through tarnish-ing layers and can remove contamination. It should be noted that this can cause abrasion, which may have a negative effect on life span and possibly contact reliability .

ƒ Use of multiple contacts (double contacts, H-contacts), with which the likelihood of good contact making is increased by parallel connection of the contact points.

ƒ Avoidance of too low contact loading and of series connection of a bigger number of contacts.

LVSAM-WP001A-EN-P - April 2009

5-10

ƒ Avoidance of interfering external influences (foreign particles, chemical effects) at the site of installation.

Fig. 5.3-10

Double contacts, increased contact pressure (for example by riffling of the contact surface) and gold-plating are some of the possible approaches to obtain good contact reliability

The attainment of satisfactory contact and hence functional reliability requires appropriate measures by the device manufacturer and the user. On the user side, the selection of a suitable contact design for the respective application from the manufacturer’s product range, compliance with manufacturer specifications and the measures listed above will have a beneficial effect on contact reliability. Care is required with all kinds of chemical substances in the switching cabinet. Thus while contact sprays may be good for oxidized sockets – for switching contacts they are poison!

Universal control contacts can be used over a wide range of voltages and powers. They are suitable both for switching contactor coils at 230 V or 110 V and for control of PLC’s at 24 V. To achieve a high degree of contact reliability, contacts should normally not be connected in series at the small control voltages as common with PLC control. With contacts that are specially designed for low signal levels it should be noted that even single switching operations at higher power levels can destroy the surface structures and hence the electronic-compatibility will completely be lost or at least strongly reduced.

Universal control contacts

Number of failures per 1 million operations

Special low-level control contacts

Number of contacts in series Fig. 5.3-11

Typical contact reliability values at 15 V/5 mA of universal control contacts and special low-level control contacts

LVSAM-WP001A-EN-P - April 2009

5-12

6 Considerations when building control systems and switchgear