Series, parallel and control circuits

Real circuits have more than one component, and the way the components are connected decides how they behave. Two arrangements cover almost everything in an elevator controller. In a series circuit the components are connected end to end in a single path, so the same current passes through all of them. In a parallel circuit they are connected across the same two points, so each has the same voltage and the current divides among them. Priya tells the class that if they understand these two arrangements, they understand the safety circuit of every elevator they will ever work on, because the safety circuit is the most important series circuit in the building.

Series circuits

When resistances are connected in series, the total resistance is their sum. The same current flows through each, and the supply voltage is shared among them in proportion to their resistances.

The sharing of voltage is the heart of troubleshooting. Kirchhoff's voltage law says that around any closed loop, the voltage rises from the supply equal the voltage drops across the components. In a series circuit of a supply and three resistors, the three drops add up to the supply voltage. A component with more resistance takes a larger share. This is the voltage divider, and the voltage across one resistor in a series string is the supply voltage times that resistor's fraction of the total.

Now consider what that means for a string of switch contacts. A closed contact in good condition has almost no resistance, so it takes almost no voltage. An open contact has, in effect, infinite resistance, so it takes all of the voltage. If a 120 V string of twelve contacts feeds a relay coil and one contact is open, a meter across that contact reads 120 V and a meter across any of the closed ones reads zero. A contact that is closed but dirty, with a resistance of a few ohms or more, takes a share proportional to its resistance and robs the coil of voltage. These three readings, zero, full voltage and something in between, are what a mechanic looks for when testing a string.

Parallel circuits

When resistances are connected in parallel, each one is connected across the same two points and has the full supply voltage across it. Each draws its own current by Ohm's law, and the total current is the sum of the branch currents. That is Kirchhoff's current law: the current into a junction equals the current out of it.

The total resistance of a parallel group is always less than the smallest branch resistance, because every branch adds another path. For two resistors, the total is their product divided by their sum. For a number of identical loads, it is the resistance of one divided by the number. A row of hall lanterns, the lamps in a car operating panel and the coils of several relays fed from the same control bus are all parallel loads. Removing one does not affect the others, and the total current tells you how many are drawing.

Parallel circuits explain why a single short circuit blows the fuse for a whole group. A short is a branch with almost no resistance in parallel with all the others, and it draws a very large current. It also explains a common surprise on older equipment: a lamp that glows dimly when it should be off. That usually means a leakage path, such as a damp fixture or a damaged insulation, in parallel with the switch that is supposed to keep it dark.

# Parallel loads on a 24 V DC lamp and indicator bus in a car operating panel
supply = 24.0
loads = {  # name: (number, resistance of one, ohms)
    "button lamps": (14, 1200),
    "position indicator": (1, 96),
    "hall lantern relays": (4, 480),
}
total_i = 0
for name, (n, r) in loads.items():
    i = n * supply / r
    total_i += i
    print(f"{name:<20} {n:2d} x {r:5d} ohm -> {i:5.3f} A")
print(f"total current {total_i:.3f} A, total resistance {supply / total_i:.1f} ohm")
print(f"power from the supply {supply * total_i:.1f} W")

button lamps         14 x  1200 ohm -> 0.280 A
position indicator    1 x    96 ohm -> 0.250 A
hall lantern relays   4 x   480 ohm -> 0.200 A
total current 0.730 A, total resistance 32.9 ohm
power from the supply 17.5 W

Series and parallel as logic

Control circuits use contacts in series and parallel to make decisions. Contacts in series act as AND: every one must be closed for current to flow. Contacts in parallel act as OR: any one closed is enough. An up-direction run circuit, for example, might require the safety string AND the door lock string AND an up direction signal AND no down direction contactor energized. A car call button circuit might register a call if the button in the car is pressed OR a call is already registered and its holding contact is closed. Lesson 7 shows how relays hold themselves in, and Lesson 11 how these arrangements are drawn on a wiring diagram, but the electrical rules are the ones in this lesson.

The safety circuit

Every elevator has a safety circuit, often called the safety string: a series chain of normally closed contacts, each belonging to a device that must stop the car if it operates. The string feeds a safety relay, or on modern controllers a safety input monitored by a certified safety processor. If any contact opens, the relay drops, the brake sets and the drive is disconnected or blocked. The devices in the string on a traction elevator typically include the pit stop switch, the buffer switches if the buffers require them, the governor overspeed switch and the governor tension sheave switch, the car safety operated switch, the final terminal limits at top and bottom, the car top stop switch, the emergency stop switch in the car where one is fitted, the machine room or control space stop switch, and others that vary with the equipment.

The door lock and gate circuits are related series strings. Each hoistway door has an interlock with an electrical contact that is closed only when the door is closed and mechanically locked, and the car door has its own contact. These contacts are often wired as a separate string so that the controller can tell the difference between a safety fault and an open door. The car may not move under normal operation unless both strings are complete.

Because the string is in series, finding an open contact is a matter of following the voltage. Ray's method, which Tamsin practises in the branch training room on a demonstration board, is to put one meter lead on the common return and walk the other lead down the string, terminal by terminal, from the supply end. Every terminal before the open contact reads full voltage to common; every terminal after it reads zero. The open contact sits between the last good reading and the first dead one. A second method, measuring across each contact in turn, gives full voltage across the open one and zero across the rest. Both are done with the circuit energized, by a qualified person with the right PPE, and neither needs a jumper.

# Node voltages along a 120 V safety string, measured to common
supply = 120.0
coil = 2400.0                      # safety relay coil resistance, ohms
contacts = [("pit stop", 0.05), ("buffer", 0.05), ("governor", 0.05),
            ("car safety", 0.05), ("final limit", 35.0), ("car top stop", 0.05)]
# case 1: a dirty final limit contact; case 2: the governor switch open
for case in ("dirty final limit", "governor open"):
    rs = [(n, 1e12 if (case == "governor open" and n == "governor") else r) for n, r in contacts]
    total = sum(r for _, r in rs) + coil
    i = supply / total
    v = supply
    print(case)
    for n, r in rs:
        print(f"  before {n:<13} {v:6.1f} V")
        v -= i * r
    print(f"  at the relay coil    {v:6.1f} V (pickup needs about 85 V)")

dirty final limit
  before pit stop       120.0 V
  before buffer         120.0 V
  before governor       120.0 V
  before car safety     120.0 V
  before final limit    120.0 V
  before car top stop   118.3 V
  at the relay coil     118.3 V (pickup needs about 85 V)
governor open
  before pit stop       120.0 V
  before buffer         120.0 V
  before governor       120.0 V
  before car safety       0.0 V
  before final limit      0.0 V
  before car top stop     0.0 V
  at the relay coil       0.0 V (pickup needs about 85 V)

The first case is the subtle one. The dirty final limit contact has 35 ohms in it, small compared with the 2,400-ohm coil, so the coil still gets most of the voltage and the relay still picks up. But if the contact gets worse, or if several contacts are a little dirty, the coil voltage drifts down towards the point where the relay drops out on a vibration. That is how intermittent safety faults begin: a car that trips occasionally on a rough start, with nothing found when the mechanic arrives. A careful mechanic on a maintenance visit measures across each contact while the car runs, and a reading of a volt or two across a closed contact is a reason to clean or replace it.

Series-parallel circuits and reduction

Most real circuits combine both arrangements. They are analysed by reduction: combine any resistances that are purely in series or purely in parallel into one equivalent, redraw, and repeat until a single resistance remains. Then work back out, finding the current and voltage in each part. The method is mechanical and reliable as long as each step really is a pure series or pure parallel combination.

Grounds and short circuits

Control circuits are normally referenced to ground at one point, usually by bonding one side of the control transformer secondary or one side of the DC supply to the equipment ground. That makes a fault predictable. If an insulated conductor on the live side of a safety circuit chafes through and touches the grounded frame, the current takes the short path to ground, the fuse blows and the car stops. The design ensures that a ground fault causes a shutdown rather than a hidden bypass. If both sides of a contact were shorted to ground, however, the contact would be bypassed, and the car could run with that device open. That is why the codes require the circuits to be arranged so that a single ground or a single failed component does not make a safety device ineffective, and why mechanics never move the ground reference of a control circuit.