BS 764:1990 pdf free download

06-12-2021 comment

BS 764:1990 pdf free download.Specification for Automatic change-over contactors for emergency lighting systems.
1 Scope
BS 764 specifies d.c. and single-phase a.c. automatic double-pole change-over contactors for low voltages up to 250 V employed in conjunction with storage batteries or other alternative source of supply, to provide a means of automatically changing the current supplied to safety lights from the normal source to the alternative source when the normal source fails, and of reconnecting the safety lights to the normal source when this source is restored.
BS 764 does not apply to three-phase a.c. automatic change-over contactors.
NOTE 1 Appendix B listg information to be supplied to the manufacturer with the enquiry andlor order.
NOTE 2 The titles of the publications referred to in this standard are listed on the inside back cover.
2 Definitions
For the purposes of BS 764, the following definitions apply.
automatic change-over contactor a complete apparatus for effecting automatic change-over of the safety lights from the normal supply to the alternative supply when the normal supply fails, and for returning the safety lights to the normal supply when this supply is restored. The return operation may be effected through the agency of a relay which, if employed, is an integral part of the contactor
safety lights
lights which are normally connected to normal supply and are changed over to the alternative supply when the normal supply fails, e.g. the safety lights specified under the Cinematograph Regulations
main contactor
apparatus constructed in accordance with this specification, excepting the relay if employed. Where the complete automatic change-over contactor is intended, this is made clear in relevant clauses
a voltage dependent relay, the function of which is to cause the main contactor to change from the emergency to the normal position when the main supply voltage has recovered to a particular value
contactor coil
the shunt connected coil energized from the normal supply, which causes the main contactor either to change to. or to remain in, the normal position
relay coil
the shunt connected coil, energized from the normal supply, which causes the relay contacts to move to one of their two positions
load carrying contacts
contacts on the main contactors which in either normal or emergency conditions carry the current for supplying the safety lights
“silver” contact
a contact composed of or faced with not less than 0.4064 mm of fine silver or other material of equivalent performance
normal position of main contactor
the position of the main contactor when it is connecting the safety lights to the normal supply
emergency position of main contactor the position of the main contactor when it is connecting the safety lights to the emergency supply
dust-protecting enclosure
an enclosure constructed to prevent the ready ingress of dust
3 Characteristics
3.1 Rating
The complete apparatus shall be rated in terms of its current-carrying capacity and voltage and also in terms of frequency if a.c.
3.2 Sizes
The following current ratings shall apply:
a) l5Aat3OV;
b) 15 A. 30 A, 60 A and 100 A at voltages not exceeding 250 V.
Where a total load in excess of 100 A applies, it should be divided over two or more separate circuits, each controlled by an independent automatic change-over contactor.
4 Design, construction and marking
4.1 General principles
The apparatus shall operate on the principle that the con tacts of the main contactor are closed to the normal position by an electro-magnet or electromagnets, energized from the normal supply, the direct pull of which is the only force preventing the main contactor from closing to its emergency position under the action of gravity or springs, or both.
Every operating spring used shall be so fitted that in the event of breakage, it shall not foul the mechanism and shall not come into contact with live parts. The use of a relay is permitted in conjunction with the main contactor.
NOTE The principles above do not refer to contact springs.
The following types of contactor are beyond the scope of this British Standard:
a) automatic change-over contactors in which the connection of the normal and alternative supplies is separately effected by two distinct contactors interlocked mechanically or electrically, or mechanically and electrically;
b) contactors dependent on electrical energy to force them into or maintain them in the emergency position;
c) contactors dependent on the application of electrical energy to release them from their normal position, or any switch employing toggles, latches, triggers, locks or similar devices, toothed gearing or ratchet action, to hold them in any position.
4.2 Mounting
The contactor shall be suitably insulated and mounted on bars or a panel. The bars or panel shall be of ample stiffness and strength. Where a panel is used, it shall be of phenolic resin bonded laminate in accordance with BS 2572.
4.3 Spacing
The spacing between the various poles of the main contactor shall be adequate for the voltage and current which are to be interrupted when tested in accordance with 5.2.5.
4.4 Contacts
Contacts, including auxiliary contacts (see 4.11) shall be of such a type that they do not exert, by reason of their construction or design, any force opposing the movement of the contact in the direction of breaking contact from either the normal or emergency position.
NOTE Contacts of the knife-switch type do not comply with the above requirements, and contactors using knife-switch type contacts are therefore beyond the scope of this British Standard.
Where “silver” contacts are fitted on contactors with ratings not exceeding 30 A. the double break bridge type of contact may be used.
4.5 Contact force
4.5.1 Hinged or pit’oted contacts. With the contactor fully closed in each position the force required to pull any contact finger off the fixed contact, as measured by a spring balance attached at the line of contact, in the direction of motion of the moving contact at the moment it is moving away from the stationary contact, shall be not less than the values given in the appropriate column, 3 or 4, of Table 1.
4.5.2 Double break bridge contacts. Where the double break bridge type of contact is used, the pressure on each contact of any one pole shall be sensibly equal, and the total force exerted by the common spring, with the contactor fully closed in each position, shall be not less than the values given in column 5of Table I.
4.6 Change-over voltage
4.6.1 The main contactor shall operate so that the poles of the safety lighting load are automatically disconnected from the normal supply and connected to the alternative supply when the voltage of the normal supply has failed or has fallen below 60 ‘o of the nominal voltage of the normal supply with the contactor coil at its normal working temperature.
4.6.2 The automatic changeover contactor (including relay if any) shall require not more than 85 % of the nominal voltage of the normal supply to bring about reconnection of the safety lights to the normal supply.
4.6.3 Operation of the contactor in accordance with 4.6.1 and 4.6.2 shall be proved by testing in accordance with 5.2.7 and 5.2.8.
4.6.4 The contactor shall remain fully closed in either position as determined by 4.6.1 and 4.6.2 until changeover occurs and the contactor changes to the alternative fully closed position. The pressure between the contacts, closed in the above manner, shall be maintained at a value not less than that sI)ecified in 4.5.1 or 4.5.2 whatever the voltage of the normal supply may be. i.e. no matter how slowly the voltage of the normal supply may rise or fall, there shall not, in either position, be a weakening of contact pressure below the minimum required by 4.5.1 or 4.5.2, as appropriate.
4.7 Sequences of operation
Both poles of the supply shall, by the action of the main contactor, be disconnected from the safety lights before these are connected to the alternative supply, conversely, it is essential that the safety lights be disconnected from the alternative supply before being reconnected to the normal supply.
5.2.4 Temperature rise tests General. Tests shall be carried out to prove compliance with 4.14. The methods to be used for measurement of ambient temperature and temperature rise are detailed in Appendix A. Temperature rise tests on cont actors and conductors. Tests shall be carried out on main current-carrying parts to prove compliance with 4.14.2 as follows:
a) for d.c. contactors with a d.c. supply at any convenient voltage;
b) for ac. contactors with an a.c. supply at any convenient voltage at rated frequency.
Connections shall be of reasonable size and a suitable interval for cooling shall be allowed after the mechanical endurance test. Tests on cant actor or relay coils. Voltage tests shall be carried out on shunt coils to prove compliance with 4.14.3 as follows:
a) for d.c. contactors with a d.c. supply at the rated voltage;
b) for a.c. contactors with an a.c. supply at the rated voltage and frequency.
The following particulars shall be recorded and held available:
1) current in the coil at room temperature:
2) ohmic resistance of the coil (in d.c.) at room
tern pe ra tu re;
3) final temperature rise.
5.2.5 Making and breaking capacity test The contactor shall make and break satisfactorily using 1.5 x rated current at rated voltage for 50 cycles at intervals not exceeding lOs on a noninductive load with both supplies connected under either condition a) or b) below. Enclosures of metal or partly metal shall not be directly earthed but shall be connected to one pole of one supply through a non-inductive resistance of 0.5 fl (approximately) in series with a fine-wire fuse, wired with copper wire not greater than 0.1219 mm diameter with a break not less than 76.20 mm.
The enclosure shall be connected to the negative pole where a d.c. normal supply is used, or to either pole where a normal a.c. supply is used. The pole of the contactor having the smallest clearance to earth shall be connected to the pole of the normal supply which is not already connected to the case (see Figure 1).
a) Where both poles of both supplies are intended to be insulated from earth, there shall be no connection between the two supplies on test.

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