1. circuit
ideas
Three-Phase Appliance Protector
 R.G. Thiagaraj Kumar and four poles.
P Kasi Rajan
. Relays RL1 and RL2 act as a
edi
sensing devices for phases Y and s.c. dwiv
M
any of our costly appliances B, respectively. These relays are
require three-phase AC sup- connected such that each acts as an
ply for operation. Failure enabling device for the subsequent
of any of the phases makes the appli- relay. Therefore the combination of mately four seconds, which energises
ance prone to erratic functioning and the relays forms a logical AND gate relay RL3 and its N/O contact closes
may even lead to failure. Hence it is connected serially. to connect the line to the energising
of paramount importance to moni- The availability of phase R en- coil of four-pole contactor relay RL4.
tor the availability of the three-phase ergises relay RL1 and its normally- Contactor RL4 closes to ensure the
supply and switch off the appliance opened (N/O) contacts close to availability of the three-phase sup-
in the event of failure of one or two connect phase Y to the input of ply to the appliance.
phases. The power to the appliance transformer X2. The availability of The rating of contactor RL4 can be
should resume with the availability of phase Y energises relay RL2 and its selected according to the full-load cur-
all phases of the supply with certain N/O contacts close to connect phase rent rating of the appliances. Here the
time delay in order to avoid surges and B to the input of transformer X3, thus contact current rating of the four-pole
momentary fluctuations. applying a triggering input to timer contactor is up to 32A. The availability
The complete circuit of a three- IC NE555 (IC1). of phases R, Y and B is monitored by
phase appliance protector is described Therefore the delay timer built appropriate LEDs connected across
here. It requires three-phase supply, around NE555 triggers only when the secondary windings of transform-
three 12V relays and a timer IC NE555 all the phases (R, Y and B) are avail- ers X1, X2 and X3, respectively. Hence
along with 230V coil contactor having able. It provides a delay of approxi- this circuit does not require a separate
8 8 • au g u s t 2 0 0 8 • e l e c t ro n i c s f o r yo u w w w. e f y m ag . co m

2. circuit
ideas
indicator lamp for monitoring the phases by disconnecting the power mounted on the backside of cabinet.
availability of the three phases. When supply through the contactor and Connect the appliance through exter-
phase R is available, LED1 glows. automatically restores the three-phase nal wires.
When phase Y is available, LED2 supply to the appliance (with reason- Caution. To avoid the risk of elec-
glows. When phase B is available, able time delay) when all the phases tric shock, ensure that AC mains is
LED3 glows. are available. disconnected during assembly of the
The main advantage of this protec- Assemble the circuit on a gen- circuit and double check everything
tor circuit is that it protects three-phase eral-purpose PCB and enclose in a before connecting your circuit to the
appliances from failure of any of the cabinet with the relays and contactor mains. 
w w w. e f y m ag . co m e l e c t ro n i c s f o r yo u • au g u s t 2 0 0 8 • 8 9