BS/EN 50556-2018 pdf download.Road traffic signal svstems.
In case of electrically isolated circuits: the insulation between the live parts of these circuits shall be calculated for the higher operating voltage.
51.5.2 Linked circuits
In case of circuits linked in an electrically conductive manner and with different voltages: the calculation of the reference voltage for the insulation between live parts shall be orientated to the highest operating voltage.
5.1.5.3 Insulation paths
5.1.5.3.1 General
For insulation paths EN 60950-1 applies, clearance and creepage distances and thickness of insulation see EN 60950-1:2006, 2.10.
5.1.5.3.2 Live parts and bodies
In addition to reliable electrical isolation of live parts in any type of circuit from live parts in power circuits, insulation paths shall be dimensioned for the following criteria.
Between live parts and bodies, the insulation paths of equipment for road traffic signal systems in an enclosure (cabinet) in accordance with 5.1.1,4 shall be dimensioned for overvoltage category II and degree of soiling 2.
Overvoltage category II: it should be ensured that no overvoltage  2 500 V can be applied, otherwise protection measures are necessary.
Degree of soiling 2: it should be ensured that in general no conductive soiling can be applied (EN 60664-1).
5.1.5.3.3 Live parts
Insulation paths between live parts of equipment for road traffic signal systems in an enclosure (cabinet) in accordance with 5.1.14 shall be dimensioned in accordance with EN 60950-1:2006, 2.10.3.3.
All terminals and other connection elements for incoming and outgoing cables or wires whose operating voltage is in excess of the extra-low voltage range shall be dimensioned in accordance with overvoltage category Ill and degree of soiling 3 (e.g. the output connector blocks in controllers, the distributors in the poles and the connection terminals in the signal heads).
5.1.5.4 Insulated devices
On equipment with an insulating enclosure and if reinforced insulation is used, the test voltage between live parts to bodies or to metal films mounted on the outer faces of insulating enclosures shall at least have double the values of the rated voltage. Insulation paths between live parts and bodies shall be dimensioned for overvoltage category Ill and degree of soiling 3 for the rated voltage.
When the cabinet is in accordance with P54 and an overvoltage protection is provided the insulation paths may be dimensioned in accordance with overvoltage category II and degree of soiling 2.
Pole distributors are considered to have reinforced insulation if the test voltage between live parts and bodies has at least three times the value of the rated voltage and the insulation paths are dimensioned In accordance with overvoltage category Ill and degree of soiling 3 for three times the rated voltage.
5.2 Traffic safety
5.2.1 General
For the present subclause concerning rules for the functional safety of Road Traffic Signal Systems the definitions for ‘item” and “failure according to IEC 60050-192 shall be applied.
Failures shall be considered at different levels of the Road Traffic Signal System. Failures considered In EN 12675 are apparent to road users. I.e. traffic and pedestrians. In this subclause those failures of items of the Road Traffic Signal System which may lead to the faults described in EN 12675 are considered.
This subclause defines the requirements for traffic signal safety, the testing of which is defined in Clause 6,
5.2.2 Requirements of signal intensity for safety
The signal limits for safety shall be to one of the following classes:
Class AFI:
The luminous intensity of the light output on axis for signals which are required for safety to be “ON, e.g. Red, shall be greater than 10 cd (as specified in EN 12368).
The luminous intensity of the light output for signals which are required for safety to be HOFF. e.g. Green, shall be less than 0.05 Cd (as specified in EN 12368).
The signal manufacturer shall specify the signal requirements either voltage, current or alternative signalling system to ensure that the above limits are met. The system engineer shall ensure that the controller shall provide an action on these limits.
Class AF5:
For signals which are required for safety to be 0N and for which monitoring of absence is required, signals shall be considered to be switched ONM if the voltage on the output of the controller is greater than minimum switch on voltage for the class of signals defined as compatible. In addition a method of confirming that the signals are drawing power (i.e. there is something connected) shall also be provided. This may be either monitoring of the current consumed by the signals or other similar technique. Any such technique shall consider the environmental effects on measurements and this shall be demonstrated in the technical assessment of the technique.
For signals which are required for safety to be OFF” shall be considered to be switched off if the voltage on the output of the controller is less than 20 % of the full rated output voltage.
5.2.3 Requirements for signal states.BS/EN 50556-2018 pdf download.