CT secondary open and PT secondary short

Why the CT secondary must not be left open

• A CT primary is in series with the line, so its current I_p is fixed by the load, not by the CT secondary.
• Normally, secondary ampere-turns N_s·I_s nearly cancel primary ampere-turns; only a small exciting current magnetises the core and the flux density is low.
• If the secondary is opened, there is no opposing m.m.f. The entire primary current becomes magnetising current, so the flux rises to saturation.
• Consequences:
  1. Very high secondary voltage: the large secondary turns and rapid flux change (flat-topped flux, steep reversals near current zero) induce peak voltages of several kilovolts. This is dangerous to persons and can break down the secondary insulation.
  2. Core overheating: high flux causes large eddy-current and hysteresis loss, which can burn the insulation.
  3. Residual magnetism: the core may keep remanent flux, increasing future ratio and phase errors.
• Therefore the CT secondary must always be connected to a burden or short-circuited (using a shorting link or test block) before removing a meter or relay.

Why the PT secondary must not be short-circuited

(The correct practice is that the PT secondary must not be short-circuited.)

• A PT primary is connected across the line and works like a normal power transformer with a fixed primary voltage; the secondary feeds high-impedance loads and runs almost on open circuit.
• If the secondary is short-circuited, the only limit to current is the small leakage impedance of the windings, so a very large short-circuit current flows.
• This overheats and burns the windings within a short time and can damage the PT and cause a fault on the HV side.
• Hence the PT secondary must be left open (no-load condition is safe) and is protected by fuses or MCBs; the PT primary is also protected by HV fuses.