This document describes a patent for improvements in protective remote control pilot circuits. Specifically, it details an electrical circuit that uses relays to control a contactor that regulates power to a remote electrical device. The circuit ensures the device cannot be restarted after a fault or supply interruption until the faults are cleared and the control switch is operated in a predetermined sequence of positions - moving it to "off" before returning it to "run".

1. * GB780063 (A)
Description: GB780063 (A) ? 1957-07-31
Improvements in protective remote control pilot circuits
Description of GB780063 (A)
PATENT SPECIFICATION
Inventors: KENNETH WILLIAM CHANDLER and BENJAMIN ROSTONDate of filing
Complete Specification Aug. 18 1954.
Application Date Mlay 18, 1953.
780,063 No. 13921/53.
Camp ete Specification Published July 31, 1957.
Index at acceutance:-Class 38(5), K(1F: 20).
International Classificat.on -H02d.
COMPLETE SPECIFICATION
Improvements in Protective Remote Control Pilot Circuits We, COAL
INDUSTRY (PATENTS) LIMITED, a company oganised in accordance with the
laws of Great Britain, of Hobart House, Grosvenor Place, London,
S.W.1, do hereby declare S the invention, for which we pray that a
patent may be granted to us, and the method by which it is to be
performed, to be particularly describd in and by the following
statement: -
The present invention relates to electrical protective remote-control
pilot circuits for the control of electrical devices and particularly
to circuits ensuring that after the occurrence of specified faults and
circuit conditions a restriction is imposed on the controlled device
which causes to to stop and/or prevents it from being restricted until
the said faults and circuit conditions are removed and until the means
of remote control have been operated in a predetermined manner.
The invention is particularly suitable for systems in which an
electrical device such as a motor is fed with current through a cable,
the current being supplied through switch gear (hereinafter referred
to as the contactor) which is dispozed at the supply end of the cable
and which is closed by operation of a control switch disposed adjacent
the electrical device, said control switch being connected through a
pilot circuit to means controlling the switch gear.

2. Such systems are usually arranged with safety means to render the
switch gear inoperative if a fault occurs in the cable or pilot
circuit or if the current supply fails; even when the supply has been
resumed or the fault repaired the switch gear remains off until the
control switch has been returned to its " off " position preparatory
to starting the motor in the normal manner.
In order to prevent the restarting of a motor after the supply has
failed and is restored, or after a fault has been cleared, until the
switch at the motor end has been brought back to the "off " position,
the remote control switch is often arranged to have three postions-"
off ", 4. " start " and " run ", in the run " position an impedance
being inserted in the pilot cir[Price 3s. 6d.] cuit at the switch end.
This impedance reduces the pilot current to a value sufficiently low
to prevent the operation of pilot devices actuating the contactor, and
thus preventing the restarting of the motor in the "run" position when
electric supply is restored. It is necessary first to cut out the
pilot impedance by moving the switch to the " start " position before
returning it to the " run " position. Thus the operation of this class
of circuits essentially depends on the magnitude of the pilot current,
and, therefore, on the critical adjustment of pilot circuit relays to
operate at the correct pilot current, and on the impedances of circuit
components which often cannot easily be maintained within sufficiently
close limits. As a result, circuits of this class often suffer from
instability and difficulty of adjustment when normal manufacturing
tolerances are present. 65 For this reason another class of circuits
has been developed, which does not essentially depend on the insertion
of an impedance when the control switch is in the " run " position,
and, therefore, on the magnitude of pilot 70 current. A mechanical
time delay switch and a relay operated by the motor current or voltage
are usually provided at the motor end of the cable where the control
switch is situated.
The contacts of the relay and of the timedelay switch are connected in
parallel. The time-delay switch is mechanically coupled to the control
switch and opens a few seconds after the operation of the latter. By
that time the relay should have operated and closed the go circuit. In
case of voltage failure the relay opens and to re-start the motor it
is necessary to close the contacts of the time delay switch by
returning the control switch to the " off " position and then
re-operating it. The disadvantages of this class of circuits are that
it depends for satisfactory performance on the magnitude of motor
current, voltage or power at any instant and the operation of the
motor at a light load which would reduce its current, 90 or at
momentary overload which would reduce its voltage, may cause the relay
to trip, or A t,' 780,063 alternatively a momentary surge may cause

3. the motor to operate when it should not do so.
Further the introduction of pure mechanical timing devices is often
undesirable as they are C too bulky for some applications and are
liable to be unlawfully interfered with; finally, it is often
specified that the mechanism for the prevention of re-starting of a
motor after fault must be located at the point of incoming supply and
not at the motor end of the cable where the control switch is located.
The present invention provides a protective remote control pilot
circuit for an electrical device which is supplied with electrical
power via a contactor remote from the device and connected thereto by
a cable and according to the invention the circuit comprises
electrical interlocking means for controlling actuation of the
contactor, said interlocking means being located adjacent to the
contactor and being connected over a pair of conductors (one of which
may be an earth connection) to switching means, which switching means
determines the direction of current flow through the conductors, the
conductors being included in the energising circuit of the
interlocking means which operates to actuate the contactor to energise
the device only if the switching means is operated according to a
predetermined sequence of switch positions.
It is preferred that the interlocking means should include relays
which are selectively energised by the switching means; the contactor
is actuated only if the relays are caused to function in a given
sequence by operating the switching means in a predetermined manner.
The invention will be more fully appreciated from the following
descriptive reference to Figures 1 and 2 of the drawings which
accompanied the provisional specification and
Figures 3 and 4 of the accompanying drawings. All are circuit diagrams
of embodiments of remote control pilot circuits for controlling
electrical machines in colleries.
Referring now specifically to Figure 1 a - control circuit transformer
T is arranged to feed say 10 volts nominal to a control system
comprising a set of relays indicated at A, B, C and D, the associated
contact of relays A being denoted by Al. The contacts are shown in the
positons which they occupy when there is no supply of current to the
input mains 10.
S1 to S8 are half-wave rectifiers. The relay A has dual windings a and
b having 3000 and 600 turns respectively; they are connected in such a
way that when half-wave rectified current flows along branches 11 and
12 (as determined by rectifiers S4 and S5) the effect on tC the
armatures is additive. The relay B has 3000 turns on its coil. The
relay C having say 650 turns on its coil, is fitted with a long copper
slug at its heel end and the relay D having 2000 turns has a smaller
copper slug also at its heel end. The control switch 13 is of the

4. two-way type having 'i off " and " run " positions in which current is
able to flow through rectifiers S9 and S 10 respectively. The motor
which is to be controlled is indicated at 14 and is fed through a
three-phase supply 70 cable 15 from the supply 10, a contactor having
contacts Xl, X2, X3 being interposed and being actuated by a contactor
coil X. A pilot core 16 in conjunction with the earth core 17 form
part of the control system belonging to the control switch 13 and are
conveniently incorporated with the cable 15. A variable resistor RI
with a maximum value of ohms is adjusted so that the relay B will just
pick up when the control switch 13 is 80 moved from " off " to " run "
position and the supply potential is 69% of its nominal value. A
second variable resistor R2 of 10 ohms maximum is adjusted so that the
R.M.S.
value of the current flowing through it is 85 0.145 amperes when a
short circuit is applied between the pilot and earth cores 16, 17 when
the supply potential is at 120% nominal value.
The system operates as follows. When the 90 control switch 13 is off,
the rectifiers S4 and S9 allow half cycles of A.C. current to flow
through the inner coil (a) of relay A which closes its armature. The
contacts Al thus allow current to flow through relay C and 9g operate
the contacts Cl and C2. Current will not flow through the outer
winding (b) of relay A or through relay B since the rectifiers S5 and
S9 are in opposition.
When the control switch 13 is moved to the 100 "run " position current
is prevented from flowing through the inner winding (a) of relay A
because of rectifiers S4 and S10 are in opposition. Hence the armature
of relay A opens and the contact Al returns to the position in 105
which it is indicated in Figure 1. No current now flows through the
winding of relay C but owing to its construction a delay occurs before
its armature opens allowing the following operations to occur. In this
condition the 110 rectifiers S5 and S10 allow current to flow through
the outer winding (b) of relay A and through the winding of relay B.
The armature of relay B closes if the incoming supply potential is
sufficient i.e. greater than 69% 115 nominal. This causes the contact
B1 to close making the operation of relay B independent of the
position of the contact Cl. The current flowing in the outer winding
(b) of the relay A is not sufficient to close its armature. 1.(, Once
the armature of relay B is closed and the armature of relay A has
opened, current flows through relay D via the contacts Al and B2 (as
the contact C2 has not yet opened). This closes the armature of relay
D, and hence the 125 contacts D1 and D2. The contact D2 makes the
relay D independent of the position of C2 and the contact D1 operates
the contactor X.
After the time delay allowed for by the construction of the relay C

5. its armature opens and 1 3 C2 and D2 are then in the position
indicated in Figure 1, and since the armature of relay is closed
operating B2 a permanent path from pilot to earth has been set up
which locks he circuit in this conditon. 70 To release the circuit
from this condition he fault resistance must be removed and action
must be taken to cause all the relay Lrmatures to open, and the
circuit will behave as under restoration of normal supply potenial,
normal operating conditions being resumed only after the control
switch has been placed in the " off " position.
If due to leakage between power cores and the motor casing and
development of a faulty S0 earth line (or for some other reason) a
potential of 30 volts or more above earth is attained on the earth
line at the remote end while switch 13 is in the " run " position,
then sufficient operating current flows through the 95 outer winding
(b) of relay A to close its armature. The circuit then operates as for
a pilot to earth fault. As soon as the contactor opens the fault
potential will be removed (were it derived from the incoming supply)
but the 90 circuit will be locked in this condition (i.e.
contactor open) by the pilot to earth connection via the contacts C2,
D2 and B2.
The variable resistor R2 was adjusted so that with the incoming supply
potential at 95 120% of its nominal value and with a short circuit of
zero or finite resistance between pilot and earth cores the pilot
current was less than 0.145 amps. This was so even before the
conducting path between pilot and earth 100 via the contacts C2, D2
and B2 was set up, after which the pilot core current is considerably
reduced.
Each relay in this circuit operates functionally during each normal
cycle of use and no 105 relay is required to operate only on
occurrence of a " rare " fault. Thus the relays are frequently tested
in normal use.
Similarly the outer winding (b) of relay A (which operates the
armature of relay A only 110 in condition (8) referred to hereinafter)
is in series with a functional part of the circuit namely the coil of
relay B and an open circuit in the winding (b) would result in the
deenergising of relay coil B and consequently if 115 relay coil D so
that the armature of D would open or remain open, and thus the
controlled apparatus would not be energised.
It is to be appreciated that the circuit of Figure 1 has been
specifically designed to 120 meet the following requirements which
have been suggested in the mining art(1) The remote control circuit,
starting with the remote control switch in the " off " position, shall
operate, in the correct sequence of 125 operations for starting, at
voltage down to 70% of the declared voltage of the incoming supply to

6. the apparatus; (2) With the control switch in the " run " position,
the remote control circuit shall conthe contact C2 changes back to the
position in C which it is indicated in Figure 1. it When the control
switch 13 is switched off the rectifier S9 prevents current flowing fi
through the path 12 and permits it to flow t through path 11. The
armature of relay B opens and that of relay A closes. The current t is
prevented from flowing through the winding a of relay D when either of
the contacts B2 or a Al changes over. A slight time delay before the
armature of relay D opens is introduced by t its construction. This
ensures that a short i circuit does not occur between pilot and earth
I (via the contacts C2, D2 and B2) should the armature of relay D
release (owing to the 1 armature of relay A closing) before relay B
releases. Since the armature of relay A is closed and that of relay B
is open, current flows through relay C and closes the contacts Cl and
C2. This leaves the circuit once again ready to operate when the
remote switch is moved to the " run " position.
In the event of a break in the pilot circuit or a lowering of the
incoming supply potential below 45% of its nominal value (when switch
13 is in the " run " position) there is insufficient current through
relay B to hold its armature closed and hence it opens, changing over
the contact B2. This releases the armature of relay D which separates
the contact DI and the contactor opens. All relay armatures are thus
open. If switch 13 is in the " off " position when the break occurs
relay A releases and thus whether the incoming supply fails either in
" off " or in " run " position all the relay armatures will be
released.
When a broken pilot circuit is repaired, or the supply to the whole
system is restored the action is as follows. When the remote switch is
in the " run " position, the armature of relay A will not close on
completion of the broken pilot circuit or on restoration of the
incoming supply or its increase in potential from a low value (i.e.
less than 45 % nominal) to any value (even over 120% nominal) since
the rectifiers S10 and S4 are opposed. The armature of relay B will
not close since the contacts BI and Cl are open. The armature of relay
C will not close until the armature of relay A closes and thus the
relay D is prevented from closing the contactor. Normal operation will
be resumed when the remote switch is placed in the " off " position,
when relays A and C will become operated.
When a resistance of the order of 50 ohms or less occurs between the
pilot and earth cores (with the control switch in either the off i" or
" run " position) current flows through the coils in both branches (11
and 12) (see Figure 1) closing the armatures of both relays A and B.
No current flows through the winding of relay C in this condition and
if not already open its armature opens; also the armature of relay D,

7. if not already open, is caused by contact Al to open. The contacts
780,063 tinue to operate at voltages down to 552; of the declared
voltage and under no condition shall continue to operate at 20%IO or
under of the declared voltage of the incoming supply to the apparatus;
(3) With the remote control switch in the "run " position, when there
is for any reason an interruption and restoration of supply to the
apparatus, the remote control circuit shall lo not energise the motor
at below 120% of the declared voltage of the incoming supply to an
apparatus; (4) The remote control circuit shall cause the contactor to
open, and remain open, on the occurrence of a pilot to earth fault of
a value which would prevent compliance with any of these conditions;
(5) The maximum current with an impedance of 1 ohm connected between
the pilot and earth terminals of a gate-end box shall not exceed 0.15
ampere R.M.S. up to 120% of the declared voltage of the incoming
supply to the apparatus; (6) The remote control circuit shall comply
with all requirements with a gate-end box at an angle of 15 in any
direction from its normal upright position; (7) All conditions shall
be satisfied with any external cable, pilot and earth, lead and
return, resistance from 0 to 3 ohms; (8) Means shall be provided to
open the contactor, and prevents its reclosing, in the event of a
phase to earth fault, in any apparatus supplied from a gate-end box,
which would raise the potential of the motor casing to 50 volts or
over above earth.
Figure 2 satisfied all the above requirements except (8), in place of
which the following requirement applied: Means shall be provided to
open the contactor, and prevent its closing, when the resistance in
pilot circuit increases by more than about 200 ohms.
Referring to Figure 2, the modified system has three relays E, F and G
and rectifiers S11-S18, the parts otherwise having the same main
references as in Figure 1. The relay F has a copper slug at its heel
end to produce the requisite delay in release. The variable 5)
resistor R3 of 100 ohms maximum is adjusted so that relay F will just
pick up when the remote switch is in the " off " position and the
supply potential is 69%,' of its norminal value.
Variable resistor R4 of 20 ohms maximum is 5M adjusted so that the
R.M.S. value of the current flowing through it is 0.145 ampere when a
short circuit is applied between the pilot line 16 and earth core 17
at a time when the incoming supply potential is at 120% nominal ht
value.
The circuit operates as follows. When the control switch 13 is in the
" off " position the rectifiers SlI and S17 allow current to flow
through the winding of relay G and close its 6S armature while that of
relay E is open since the rectifiers S14 and S17 are opposed. Current
flows through the winding of relay F via the contacts G1, El and G2

8. and closes its armature. This closes the contacts F1 and renders the
armature of relay F independent 70 of the contact G2.
When the control switch 13 is moved to the "run" position the armature
of relay G is released, since no current flows through its winding,
the rectifiers SlI and S18 being 7? opposed. Since the rectifiers S14
and S18 are not opposed the armature of relay E closes provided the
switch operating time is shorter than the release time of relay G.
During the period of these changes no current is supplied to the 80'
winding of relay F but owing to its construction its armature will
remain closed for this short period. When these changes are complete
the winding of relay F is again supplied with current via the contacts
El, GI and F1 85 and its armature continues closed. In this condition
the contacts E2, F2 and G3 are closed which causes the contactor to
close and supply current to the controlled gear.
When the control switch 13 is returned to 96 the " off " position the
armature of relay E is released and that of G closes. The contacts E2
and G3 open and the contactor switches off the controlled apparatus.
If the incoming supply potential is lower 95 than 69%i of its nominal
value when the position of the remote switch is changed from " off "
to "run" the armature of relay F will not remain closed during the
changeover period. The contacts F2 and Fl also open, lot which means
that the armature of relay F will only close again when the supply
potential is above 69% of its nominal value and the remote switch is
again in the " off " position (i.e. armature of relay E is open and
that of G 105 is closed).
When the supply fails, or the pilot circuit is broken or its
resistance rises to a fairly high value (of the order of 10-200 ohms)
both the armature of the relays E and G are open and 110 the winding
of relay F receives no current and its armature also releases. This
switches off the contactor. The circuit remains thus until the
conditions revert to normal and control switch 13 is moved to its
"off" 115 position.
When the incoming supply potential falls below 50% of its nominal
value with the remote switch in the "run" position, armature of relay
F is released and the conditions 120 are similar to those for a drop
in the supply potential below 690% before switch 13 is moved to the "
run " position.
In the event of the application of resistance of the order of 50 ohms
or less between the 125 pilot and earth cores of the trailing cable,
the armatures of both relays E and G are closed.
Thus contact G3 is open and the contactor switches off the supply to
the controlled apparatus. No current is supplied to the 130 780,063
780,063 winding of relay F, in this condition, and its armature
releases after the short time interval allowed for in its

9. construction. The armature of relay F will only close when the normal
conditions are restored, and the control switch is off. In the course
of such a short circuit the pilot current is limited to 0.145 amps. by
the resistor R4, and the circuit satisfies condition (5) stated above.
The rectifiers connected across relay winding in both circuits
(Figures 1 and 2) are required in order to reduce the danger of
inductive surges causing ingnition of methane air mixture. In addition
they tend to stabilize the performance of the relays.
Referring now to Figures 3 and 4 two circuits are shown using remote
control pilot circuits. The circuits satisfy the following
requirements: i. The remote control circuit, starting with the remote
control switch in the " off " position, shall operate, in the correct
sequence of operations for starting, at voltage down to 75% of the
declared voltage of the incoming supply to the apparatus; ii. With the
control switch in the "run " position, the remote control circuit
shall continue to operate at voltages down to 55% of the declared
voltage and under no condition shall continue to operate at 20% or
under of the declared voltage of the incoming supply to the apparatus;
iii. With the remote control switch in the " run " position, when
there is for any reason an interruption and restoration of supply to
or from the apparatus the remote control circuit shall not energise
the motor at below 120% of the declared voltage of the incoming supply
to an apparatus; 4C iv. The remote control circuit shall cause the
contactor to open and remain open on the occurrence of a pilot to
earth fault of a value which would prevent compliance with any of
these conditions; v. The maximum current with an impedance of 1 ohm
connected between the pilot and earth terminals of a gate-end box
shall not exceed 0.25 ampere R.M.S. up to 120% of the declared voltage
of the incoming supply to the apparatus; vi. All conditions shall be
satisfied with any external cable, pilot and earth, lead and return,
resistance 0 to 3 ohms; vii. The remote control circuit shall be
prevented from " starting " if at nominal volts the external cable,
pilot and earth circuit and return resistance exceeds 23 ohms; viii.
The remote control circuit shall comply with all these conditions with
the gate and box at an angle of 15 in any direction from its normal
upright position.
In Figure 3 the transformer T is arranged to feed voltage (6 volts) to
the relays A.B.C.D.
Half wave selenium rectifiers are denoted by Figures 1-10.
Relays A.B.C.D. are constructed as shown in tabular form below.
Relay A has 2500 turns and is provided with a small heel end slug; B
has 1500 turns and is provided with a 70 small armature end slug; C
has 1250 turns and is provided with a long armature end slug; D has
1500 turns and is provided with a small armature end slug. 75 It will

10. be seen that the control switch has two positions-" off " and " run ".
The modus operandi of the circuit is as follows: When the control
switch is " off ", the rectifiers 5 and 9 allow current to flow
through relay A which closes its armature. Current does not flow
through relay B (the rectifiers 7 and 9 are opposed) and contact B1 is
not operated. The contacts Al thus allow current to 85 flow through
relay C and operate the contact Cl.
Consider now the case when the control switch is moved to the " run "
position, current is prevented from flowing through A (5 90 and 10
opposed). Relay A opens and contact Al returns to the off position.
Current, however, does flow through relay B (7 and 10) and the contact
B11 operates (if the voltage is sufficient and if the pilot resistance
is less than 95 23 ohms, requirements i and vii). No current flows
through relay C but a delay occurs before its armature opens owing to
its special construction. Providing relay B has operated then relay D
is permitted to operate through contact Al (off) contact B1 (on) and
contact Cl (delayed release); contact D1 then makes the operation of
relay D independent of contact e-1. Contact D2 operates the coil of
contactor X and after the delay C releases. 105 - Consider now the
following conditions:On switching " off " relay A closes, relay B
releases, relay D releases and relay C closes.
The circuit is ready to operate again.
On a break in the pilot circuit (with switch 110 in " run " position)
or lowering of the supply potential below 55% of its nominal value
there is insufficient current to hold relay B. Contact 131 changes
over, relay D releases and D2 opens, contactor opens. Similarly if
control switch is " off " relay A releases causing relay C to release.
All relays are then unenergised. Simifarly if incoming supply fails.
When a broken pilot is repaired or supply 120 restored etc., with the
remote switch in the " run " position relay B closes and contact B1
changes over; relay D is unable to close since contact Cl open (C not
previously energised); Relay A will not pick up (5 and 10 opposed) 125
and relay D is therefore prevented from operating the contactor.
Normal operation resumed when switch placed in " off " position, when
relay B will release and relays A and C become energised. 130 When a
resistance of the order of 30 ohms or less occurs between pilot and
earth (with the switch either in the " off " or " run " positions)
both relay A and relay B will be energised, contacts Al and B1 will be
in the operated position, and relays C and D, if not already released,
will release.
On removing the resistance with the switch in the "run" position relay
A releases and it, relay B remains energised, and Al goes to
unoperated position. There is no path for relay D to pick up since
contact Cl is open (C not previously energised). If the switch is in

11. the " off " position relay B would release I and relay A remains
energised permitting relay C to close, ready for normal operation.
On a 1 ohm short occurring between pilot and earth when the voltage is
120% nominal the current in the pilot is less than 0.240 amperes. Thus
the circuit complies with condition V.
In Figure 4 the circuit has 3 relays A, B and C, which are constructed
as A, B and C of Figure 3. The system operates as before, is relay D
being replaced by the contactor coil, A1 by A1 and A2, Bl by B1 and
B2, and D1 by an auxiliary contact X4 on the contactor.
Since the contactor is energised by a medium voltage AC, the circuits
for relay C and for the container are separate.
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