Technical information

CONTROL CIRCUITS

1.4 Special prescriptions for Control Circuit class 1

If the control circuit is derived downline from a branch circuit protection that

already exists, an overcurrent protection should

be installed at the point of

derivation of the control circuit conductors.

For the equipping of general machines UL508A it is required to install a

protection, sized in accordance to the general rules previously listed.

If,however,the circuit has been derived in a motor branch circuit two e xceptions

are admitted:

1. if the control circuit is still confined inside the control panel (for example the

command push buttons are mounted on the metal work), it is protected from

over currents by the BCP motor (without additional protections) if the values

shown in table 41.1 of UL508A are respected.

2. if the control circuit leaves the panel (for example in the case of remote

pushbutton control station) it is protected against the over currents by the

motor BCP (without additional protection) if the values defined in table 41.2

of UL508A are respected.

Standard CSA 22.2 #14 also makes a similar exception: if the control circuit

leaves the panel it is not necessary to have an additional overcurrent

protection if the motor BCP has a rating that is no more than 300% the

capacity of the conductors that, in their turn, should not be lower than 15A.

In the case of Industrial Machinery the NFPA 79 standard (and the relative

section of UL508A) require the over current protection always to be installed and

the sizing not to be a bov e the value in table 66.4 of UL508A on the basis of the

control circuit conductor cross-section, the BCP and the positioning of the

components connected to the circuit.

In this regard it should be remembered that the control circuits can be derived

directly (down line of an already existing BCP) only from the power circuits with

voltage no more than 120 V.