THE AMAZING SERIES TEST LAMP

The Electric circuit is continuous in nature, It means it has entry & exit path for current flow.
(NOTE: In this post, We are defining wire bind elements as an electric circuits such as inductive coils & resistive coils)
Healthy circuit is complete & continuous without any type of breakage or leakage. There are multiple methods to detect Whether circuit is healthy or not.
Continuous & completeness of the circuit, is represented with the term ‘Continuity‘.

Continuity

“Continuity is the presence of complete path of a circuit; without any breakage; Where current flows. Closed & complete circuit has ‘continuity’.”

If there is two terminals inductive coil with end terminals A&B and it is complete without any breakage than we can say; terminal A is continuous with terminal B or we can say coil is having a continuity.

In Fault conditions, the continuity of coil get break either it get burnt or earthed. So that, current don’t have continuous defined path to flow which results; circuit don’t work.

In a GIF below, Continuous circuit is shown. Two terminals coil; having a terminals A & B; is connected to a battery. Batterie’s positive terminal is connected to terminal A While negative is connected to terminal B.  Current starts flowing from positive terminal to negative terminal. That circuit represents the continuous circuit; & Having a ‘continuity’. 

GIF shows the working of continuous circuit
                                                    Working of Continuous Circuit

In a GIF below, Continuous circuit is get disconnected. At starting, When circuit is continuous, Current flows from terminal A to terminal B. Due to some circumstances, Coil get break & circuit get discontinuous. Current stops flowing.

Gif shows the working circuit get discontinuous.
                                                       Working Circuit get discontinuous

METHODS TO FIND OUT; CONTINUITY

To find out the continuity; Multimeters can be used Which contains a special option of continuity. In which we just need to select the continuity option & than by placing both the test leads across the testing element, Continuity can be find out. A beep sound or some resistance value reflect on the display of Multimeter.

Multimeters generally uses DC supplies at low voltage for continuity. Sometimes, It is difficult to find out the continuity through multimeter When terminals of the circuit are carbonized or When circuits resistance is much higher, Higher than the range of multimeter.

There is another method for continuity testing without multimeter & that is ‘Series test lamp method’. In this method, consumer level distribution voltage can be used.

Sign of continuity| Mastech Clamp meter M266
Sign of continuity printed on Mastech Clamp meter M266

Series Test Lamp method

Series Test Lamp is an arrangement of two terminals or two test leads in which one terminal is phase(ex. 230VAC) & another terminal is neutral with a condition that phase should passes through the Bulb (Incandescent) first; In a series manner. (Shown in an image below)
When these two test terminals get shorted, bulb starts glowing with full brightness. With this test lamp method, Continuity can be find out easily.

Main Components of a Series Test Lamp

  • Incandescent bulb (60W / 100W)

  • Bulb holder

  • Two connecting wires

  • Plug top

  • Insulated board or casing

Circuit for series test lamp

Series test lamp circuit shown below. Neutral is directly available as a test lead While phase passes through the 100Watt bulb in a series manner.

Series TEST Lamp with 100 Watt Incandescent Bulb
Series TEST Lamp with 100 Watt Incandescent Bulb

 

Outcomes with series Test Lamp?

With series Test Lamp, We can find :
  • Whether circuit is continuous or not !
  • Whether circuit is isolated from another circuit or not !
  • Whether circuit is isolated from device body or not !
  • Whether multiple circuits are identical or not !

Bulb Indication Meaning

  • Bright glow → Short circuit / heavy fault

  • Dim glow → Normal condition

  • No glow → Open circuit / no load

 

How does a Series Test Lamp method is performed ?

Series Test Lamp has two test terminals, When we short these two terminals Bulb starts glowing with full brightness because of the full voltage drop across the bulb.
For performing Series lamp Test, We are taking an example of 3 phase Induction motor which has total 6 leads.
{3 phase Induction motor with 6 connection leads, In which 6 leads are provided so that we can choose the motor connections accordingly.
We can operate the motor in star, Delta or in star-Delta connections. These 6 leads contains 3 winding sets which means each set has 2 terminals- U1-U2, V1-V2, W1-W2 As shown in image below.

3 phase 2.2KW 1440 rpm Induction motor with 6 terminals !
3 phase 2.2KW 1440 rpm Induction motor with 6 terminals !
 For better winding understanding, A gif is shown below :
GIF shows 3 phase winding configuration Star & Delta
STAR DELTA WINDING CONFIGURATION
At Normal working conditions
  1. All the 3 winding sets are equally balanced & identical with each other with identical impedances.
  2. U1 should be continuous with U2, V1 should be continuous with V2 & W1 should be continuous with W2.
  3.  All the 3 winding sets U1-U2, V1-V2 & W1-W2 should be Electrically isolated from each other. 
  4.  All the winding sets should also be isolated from machine body. 
 
•Whether Circuit is continuous or not !
Place one neutral terminal of series test Lamp on U1 & another terminal of T-Lamp on U2.
If Bulb glows than circuit is continuous otherwise circuit is break.
Similarly, perform test on V1-V2 & W1- W2.
 
 
•Whether circuit is isolated from another circuit or not !
Place one neutral terminal on U1 & another terminal of T-Lamp on V1,V2,W1,W2 turn by turn.
If Bulb doesn’t glow than circuit is isolated from another Circuit otherwise circuit is not isolated.
Similarly, perform test on both the winding sets V & W.
 
•Whether circuit is isolated from machine body or not !
Place one neutral terminal on Machine body(Earthing terminal) & another terminal of T-Lamp on all the 6 terminals V1,V2,U1,U2,W1,W2 one be one.
If Bulb doesn’t glow than circuit is properly isolated from machine body otherwise not.
 
•Whether multiple circuits identical or not !
Place both the Terminals of T-Lamp on winding set U1 & U2, Observe the brightness of the bulb After then place both the terminals on V1-V2  & also on W1-W2. Observe bulb brightness on all the 3 sets.
If Bulb brightness is same on all the 3 sets than all the 3 sets are identical & balanced. If Bulb brightness is different than sets are not identical.
 

Where do we perform this test?

  • On motor windings as explained above!
  • On resistive heating coils!
  • On inductive coils like solenoid coils!
  • To check the continuity of cable or wire!
  • As an current limiting element for another electrical load.

 

Series test lamp as an current limiting Element

Any type of Electrical load whether it is single phase or three phase or whether it is light load or heavy; the voltage always remains same. It is the ‘current & power factor‘ which varies according to the load. Light load takes low current while heavy load takes heavy current.
In faults conditions specially in short circuit, Voltage decreases & current increases rapidly. It is the heavy current flow which damages the device in short circuit conditions.
If we have low wattage device such as 20Watt & we are not sure whether device is healthy or not. We can use series Test Lamp out there. With Series Test lamp(100watt incandescent), Device can get operated. If the device is healthy than device will starts working & bulb starts glowing in a dim brightness. Otherwise in fault condition such as short circuit, bulb glows with full brightness. In short circuit conditions, the bulb work as current limiting element & full voltage drop occur across the bulb.
 (One of my own experience:

“Hand/angle Grinders which works on AC series motor whose running speed can be up to 12000rpm. Due to continuous running; Mostly its carbon brushes get deteriorated.

While detecting the fault of hand/angle grinder on direct 230Volts supply, Is always dangerous because of its high starting torque & high speed. But we can easily check the hand grinder on series Test Lamp supply because at test Lamp supply, grinder rotates at very slow rate & we can easily detect which carbon brushes are working or not or which part is damaged.”)

 

Limitations of series test lamp method

  • This test needs an Voltage source. 
  • Risk of Electric shock, needed to be careful while performing this test. 
  • Sometimes, It is difficult to Distinguish the brightness on different circuits.

 

Safety Precautions

  • Use proper insulation

  • Never touch live terminals

  • Use correct bulb wattage

  • Avoid wet hands

 

Summary

  • This post discusses the concept of electric circuit continuity, emphasizing that a healthy circuit is complete and uninterrupted, allowing current to flow.
  • The significance of continuity and methods to test it, notably using multimeters and the Series Test Lamp method.
  • The Series Test Lamp can verify circuit continuity, isolation, and identity in various electrical components like Induction motors and coils.
  • The post also highlights its utility in fault conditions, serving as a current limiting device to protect against short circuits.
  • However, it notes limitations such as the need for a voltage source and risks of electric shock.

Frequently Asked Questions (FAQ)

Q1. Which bulb is best for a series test lamp?
👉 60W or 100W incandescent bulb is commonly used.

Q2. Can I use an LED bulb?
👉 No. Only incandescent bulbs work correctly.

Q3. Is a series test lamp better than a multimeter?
👉 It does not replace a multimeter but is excellent for initial fault detection.

Q4. Can beginners use a series test lamp?
👉 Yes, it is one of the best tools for beginners.

Q5. Does it save appliances from damage?
👉 Yes, it limits current and prevents serious damage.

Hope! You like the post. If you have any doubt, suggestion or query please do comments.
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