Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 55)

In applications with increased stress by shock and vibration such as for example in vehicles, in
rail transport or on ships a variety of measures is required to protect the devices from the
immediate influence of externally generated shock and vibrations. In the simplest case, by
optimization of the mounting position. In case of doubt, the manufacturer should be consulted.
2.4 Specific application conditions and switching tasks
2.4.1 Parallel and series connection of poles
2.4.1.1 Parallelling
Parallel connection of poles in switchgear increases its thermal load capacity. It should be
remembered that the resistances of the individual poles vary due to contact burn-off, deposits
etc. The current does not distribute itself equally among the parallel poles, but corresponding to
their particular impedances.
A reduction factor for the total load must be applied to avoid overloading of the individual
contacts. In practice the following values for the permissible total current can be calculated with:
- with 2 parallel poles
I
e2
= 1.8 x I
e
- with 3 parallel poles I
e3
= 2.5 x I
e
The making and breaking capacity remain in parallel circuits the same as for single contacts , as
frequently one contact is opening or closing first and therefore must take the largest part of the
switching work. Therefore it is not possible to increase the contact rating of contactors for
switching motors and capacitive loads by parallel connection of contacts.
Tab. 2.4-1 shows the
switching capacity based on the total current with 2 and 3 contacts connected in parallel.
Three
pole
switching
Æ I
e
2 poles in
parallel 1)
Æ I
e2
= 1.8 · I
e
3 poles in
parallel 1)
Æ I
e3
= 2.5 · I
e
Making capacity 12·I
e
(12·I
e2
)/1.8 = 6.7·I
e2
(12·I
e3
)/2.5 = 4.8·I
e3
Breaking capacity 10 · I
e
(10·I
e2
)/1.8 = 5.6·I
e2
(10·I
e3
)/2.5 = 4·I
e3
1) Voltage across each contact U = U
e
/ √3
Tab. 2.4-1
Making and breaking capacity of contactors as a multiple of the rated operational current I
e
for three pole
switching and for two and three parallel poles
For this reason contactor poles should only be connected in parallel for switching resistive loads
(utilization category AC-1). Where possible they should only be connected in parallel by means
of copper bars fed in the center in order to ensure symmetrical current distribution and good
heat dissipation. For small contactors special connecting bridges are available.
Any short-circuit currents that occur are distributed between the poles depending on the given
pole resistances. In the case of circuit breakers with parallel contacts, it may happen at small
short-circuit currents that the operating current of undelayed electromagnetic short-circuit
releases is not reached. Consequently such a short-circuit is only switched off by the thermal
release after a delay. The pick-up threshold for undelayed short-circuit breaking rises approxi-
mately by a factor given by the number of parallel poles.
2.4.1.2 Series connection
When two or three poles of switchgear are connected in series ( Fig. 2.4-1 ), the advantages
include the following:
- Increased dielectric withstand voltage
- Improved switching capacity
- Higher operating voltage
- Larger contact life span
LVSAM-WP001A-EN-P - April 2009 2-27