After a machine trips, the electrical technician usually goes to see which protective device operated. A leakage protector and an overload protector point to completely different root causes and are dealt with in different ways. Treating the two as the same thing leads most often to the protector being replaced and the fault returning as before, with every trip starting the search from zero.
What each of the two lines of defence watches
Leakage protection watches whether the current has departed from the normal state of going out and coming back. Under normal conditions the phasor sum of the three phase currents and the neutral current should be zero; as soon as the insulation degrades or one phase leaks to the enclosure or to earth, that sum is no longer zero and the device trips once it passes the setting. It guards against personal shock and against leakage that can start a fire.
Overload protection watches whether the current has exceeded what the machine itself can carry. It does not care which path the current takes, only how large it is. An overheated motor winding, a seized drive and mechanical drag in the oil pump will all push the running current up, and once it has accumulated to the point on the inverse time curve the breaker drops out. It guards against the machine itself being burned out.
The two act on different things: the first cuts a current path that should not exist, the second cuts a path that is normal but carries too much current. On site both look like the machine has stopped, but the starting point of the search is completely different, and mixing them up sends the search around in circles.
What the two kinds of trip look like on site
The machine carries more than these two protections. Phase failure and phase sequence protection, and the overpressure alarm that shuts the machine down, also stop the machine, but the symptoms are quite different. Setting the common types side by side makes the judgement on site much faster, and it prevents an overpressure shutdown being investigated as an electrical fault and an electrical trip being investigated as a blocked pipe.
| Type of protection | What it watches | Typical causes of operation | How it is reset |
|---|---|---|---|
| Leakage protection | The difference between the incoming and the outgoing current | Insulation ageing, damp, damaged cable, leakage between the turns of a winding | Manual reset after the insulation fault has been found and removed |
| Overload protection | The size of the current in the circuit | Long term overload, mechanical binding, low supply voltage causing a higher current | Reset per procedure after cooling down; for a thermal device, confirm whether it resets itself |
| Phase failure and phase sequence protection | The integrity of the three phase voltage | A missing phase, reversed phase sequence | Reset after the supply problem has been cleared |
| Overpressure alarm shutdown | The pressure in the filtration section and the system | A blocked element, a bypass valve opened by mistake, a closed outlet valve | Release the pressure, deal with the pressure source, then reset through the normal sequence |
How to investigate after a trip
- First establish which protective device operated; do not infer it from the colour of an indicator lamp or from a label on the panel.
- Record the operating condition at the moment of the trip: the load state, the oil temperature, the vacuum level and whether the machine was in the heating stage.
- Measure and record the three phase currents and compare them with the rated values on the nameplate, to see whether the machine has been running close to its limit.
- Check the cables, the terminals and the earth terminal of the supply circuit for damp, oxidation or looseness.
- On the machine side, look at the insulation of the motor windings, the heater connections and the oil circuit components against the enclosure.
- If the protection keeps operating and the cause is not known, do not raise the setting to make the fault disappear, because that only postpones it.
Why this line of defence is worth separating
A leakage protection operation means the current has found a path it should not have taken, and such a fault usually carries a safety risk, so it has the highest priority. An overload protection operation means the system is still working the way it was designed to, but the load is beyond the margin, and the direction is normally to find the mechanical drag or to adjust the conditions rather than to change the electrical components.
Separating the two clearly shortens the downtime on site. Taking the ZYD-100 as an example, the total power is 72 + 5 kW and the supply is three phase 380V/50Hz, so the running current of the motor is not small to begin with; running close to the rated value over a long period is a typical trigger for the overload protection, and that is a completely different matter from a leakage fault.
Leakage protection deals with a current that should not be there, overload protection deals with a current that should not be that large; decide which one you are chasing before the search starts.
