Question

A high tension (HT) supply of, say, 6 kV must have a very large internal resistance. Why?

Answer 1

The primary reason is safety and current limiting.

Let's use Ohm's Law ($V = IR$) to understand this. A power supply can be modeled as an ideal voltage source ($Vs$) in series with its internal resistance ($R{int}$). The device you connect to it is the load resistance ($R_{load}$).

The current that flows through the circuit is given by:
$I = \frac{Vs}{R{int} + R_{load}}$

Now, consider what happens in a dangerous situation, like a short circuit or a person accidentally touching the output terminals. In this case, the load resistance ($R_{load}$) becomes very low.

Case 1: HT Supply with a Low Internal Resistance (Hypothetical & Dangerous)

Let's imagine our 6 kV (6000 V) supply had a very low internal resistance, like a car battery, say $R_{int}$ = 0.1 Ω.

If you were to short-circuit this supply, the current would be:
$I = \frac{6000 V}{0.1 Ω} = 60,000 A$

This is an apocalyptic amount of current. It would create a massive explosion, vaporize wires, and be instantly fatal. Even if you didn't short it completely, but just provided a path to ground through your body (which has a resistance of a few thousand ohms), the current would still be lethal.

Case 2: HT Supply with a High Internal Resistance (Realistic & Safe Design)

Now let's take the same 6 kV supply but give it a very large internal resistance, say $R_{int}$ = 10 MΩ (10,000,000 Ω). This is often achieved by putting a large physical resistor in series with the output.

If you were to short-circuit this supply ($R{load} \approx 0$):
$I{max} = \frac{6000 V}{10,000,000 Ω} = 0.0006 A = 0.6 \text{ mA}$

This maximum possible current is 0.6 milliamperes. This is a tiny, non-lethal current. You would feel a sharp shock, but it is not life-threatening because the high internal resistance inherently limits the current to a safe level, no matter what you connect to the terminals.

Summary of Key Reasons:

1. Personnel Safety: This is the number one reason. The high internal resistance acts as a built-in, permanent safety feature. It ensures that even if the high voltage terminals are accidentally shorted or touched, the resulting current is too low to cause serious harm or electrocution.
2. Equipment Protection: In the event of an accidental short circuit, the limited current prevents the internal components of the power supply (transformer, rectifiers, etc.) from being destroyed by a massive current surge.
3. Application Suitability: Many applications of such HT supplies (e.g., electrostatic experiments, insulation testing, Geiger-Müller tubes) require a stable high voltage under a very low current draw. They are not designed to deliver power. The high internal resistance is perfect for this, as it creates a high-voltage, low-current source.

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Important Distinction: When is High Internal Resistance NOT Desirable?

It's crucial to understand that this principle does not apply to all high-voltage systems.

For high-voltage power transmission lines (the ones on large towers), the goal is the exact opposite. They are designed to have the lowest possible internal resistance.

Why? Because their purpose is to deliver maximum power efficiently over long distances.
Power Loss: Power lost in the lines is given by $P{loss} = I^2R{int}$. To minimize loss, you must minimize the resistance ($R_{int}$).
Voltage Sag: The voltage delivered to the customer is $V_{load} = Vs - I \times R{int}$. If the internal resistance were high, the voltage would drop dramatically as soon as customers started drawing current, which is unacceptable.

So, to conclude:

• An HT supply for a lab, testing, or low-power application has a high internal resistance for safety and current limiting.
• An HT supply for power transmission has a very low internal resistance for efficiency and power delivery.