PSU Protections Explained: OVP, OCP, OTP and Short Circuit

Why Built-In Protections Matter

When a PLC or sensor fails and creates a short circuit, when an overload pulls the supply beyond its rating, or when cabinet temperature spikes — the difference between "supply tripped on a fault" and "supply destroyed the entire panel" is a set of built-in protections. Every modern industrial power supply from Mean Well, Delta, and Schneider in our catalogue carries the standard protection set: OVP, OCP/OLP, SCP, and — on most mid-range and premium models — OTP.

Here is what each protection does, when it triggers, and how the supply recovers.

OVP — Over Voltage Protection

OVP activates when the output voltage exceeds a set threshold — typically 110–135% of the rated value. Causes include a feedback loop failure, a mains transient, or accidental adjustment of the output trim potentiometer.

OVP circuits are typically implemented as a crowbar (thyristor that collapses the output) or a PWM shutdown circuit. When triggered, output voltage drops to zero or the supply shuts off completely.

Recovery: most supplies after an OVP event require a power cycle (remove and restore mains). Some series (e.g., Mean Well NDR) auto-reset after a delay. The recovery mode is always stated in the product datasheet.

For the load, OVP is critical: 30 V instead of 24 V for even a fraction of a second will destroy the input capacitors of a PLC or damage sensitive analogue I/O cards.

OCP and OLP — Over Current and Over Load Protection

OCP limits output current when the load demands more than the supply's rated current. There are three common response modes:

• Constant current limiting (CC mode). The supply holds output current at a fixed level (e.g., 105% of rated), while voltage sags. The load continues to receive power, albeit reduced. This is ideal for motors and capacitive loads where a high inrush current on start-up is normal.
• Hiccup mode. The supply attempts to restart; if the overload persists, it enters a burst pattern — short on-cycles followed by pauses. Once the overload clears, the supply auto-recovers. Hiccup is typical for LRS, HDR, and DR series.
• Fold-back (foldback current limiting). Under overload the current is not just capped but actively reduced — "folded back" — to an even lower level. This protects both the supply and the load from sustained thermal stress during a prolonged short circuit. Common in some NDR and RSP models.

OLP is a separate term for overload power protection; it is functionally equivalent to OCP and often implemented by the same circuit.

SCP — Short Circuit Protection

SCP is the supply's ability to survive a direct output short circuit without damage. It is often described alongside OCP — in hiccup mode, the supply withstands a short of indefinite duration until the circuit is cleared.

Short circuits in industrial automation are not uncommon: wiring errors during commissioning, insulation breakdown, actuator failure. A supply with SCP survives the fault and resumes operation once the cause is removed. A supply without SCP fails, likely taking the rest of the panel offline with it.

All DIN-rail power supplies in our catalogue (Mean Well, Delta, Schneider) include SCP. Check the datasheet for entries such as "Short circuit protection: hiccup mode, auto recovery."

OTP — Over Temperature Protection

OTP shuts down or derate the supply if internal component temperature or ambient temperature exceeds a safe limit — typically 70–90°C at a thermistor placed near the power transformer or output switching devices.

Two response modes exist:

• Full shutdown with auto-recovery. The supply switches off until it cools, then restarts automatically. Typical for NDR, RSP, and DRP series.
• Derating without shutdown. Some series gradually reduce output power as temperature approaches the limit without fully switching off — better for systems where a sudden power loss is unacceptable.

OTP is especially relevant for panels with poor ventilation or installations at elevated ambient temperatures. Regular OTP events are a clear indicator that cabinet airflow or supply power margin needs to be revisited.

Protection Summary Table

Hiccup Mode in Detail: Why It Is the Right Behaviour for Short Circuits

In hiccup mode, rather than holding the output at maximum current through a short circuit, the supply performs rapid shutdown-and-restart cycles. At each restart attempt it checks whether the fault has cleared. If not — it pauses and tries again.

Benefits of hiccup mode:

• The supply does not overheat during a prolonged short — average power dissipation in burst mode is minimal.
• Recovery is automatic once the fault is cleared — no manual reset or site visit required.
• The load does not receive a steady "background" short-circuit voltage that could damage partially connected circuits.

Constant current fold-back is preferable when the load has large capacitive inrush on start-up and a supply restart would be disruptive.

How These Protections Tie Into Panel Reliability

A supply's protection set is the first line of defence for the entire panel. A correctly selected Mean Well or Delta with the full OVP + OCP + SCP + OTP set:

1. Does not fail itself when an individual load channel overloads or shorts.
2. Does not propagate the fault to other panel components.
3. Recovers automatically once the fault is cleared — no operator intervention.
4. Signals the fault through a voltage sag, allowing the control system to log the event.

When selecting a supply, always check the datasheet for all four protections and note the OCP recovery mode — it determines whether the supply self-recovers after a short circuit is cleared or requires a manual power cycle.

Have a specific fault scenario to review? Send us your load list — we will recommend the right supply within one business day.