A loose electrical terminal is one of the faults that takes the longest to develop. It does not trip on the first day. It pushes the contact resistance up a little at a time, the joint starts to heat, the contact oxidises, the resistance rises further, and only at the end does it turn into a trip, smoke or a machine that will not start. That process can run for weeks or months, and in between what the site sees is a set of symptoms that are hard to explain.
How the contact resistance is pushed up
A sound terminal relies on the bolt pressure to press two contact faces together, and that establishes a low resistance path. Once the joint loosens the pressure falls, the faces touch at only a few points, and the current passes through those points. Concentrated in such a small area, the same current goes through a much smaller section, the temperature at the contact point rises at once, and the oxidation that follows makes the face worse. The resistance then rises further, and the process feeds itself.
The supply of the ZYD-100 is three phase 380V / 50Hz and the total power is 72 + 5 kW. The most common form of a loose terminal is an unbalanced three phase load: one phase with a higher contact resistance carries less current while the other two carry more, and the motor shows an uneven load and an uneven temperature distribution, while the trip usually happens long after the load has moved away from normal.
What traces it leaves
- The terminal itself or the insulation of the cable next to it runs hot, and a measuring device finds it long before a hand does.
- The terminal surface is discoloured or blackened, or the plastic around it has been deformed by heat.
- There is a burnt smell mixed with the smell of overheated plastic, which means overheating has already happened rather than being about to happen.
- The machine trips now and then and runs for a while again after a restart, and this intermittent behaviour points towards a poor contact.
- The motor runs with an unsteady noise and more vibration than usual, while the three phase readings on the current meter do not agree.
The order of the checks
| Step | What to do | What this step is for |
|---|---|---|
| 1 | Stop the machine, cut the power and confirm the circuit is dead | Safety, and avoiding a second fault from tightening a live joint |
| 2 | Inspect the terminals, the cable lugs and the insulation by eye | Looseness, discoloration, indentation and ageing insulation |
| 3 | Measure the temperature of each phase under load | Whether the temperature distribution leans towards one phase |
| 4 | Retighten to the specified torque and replace terminals that have deformed | Whether the contact face is complete and whether the fastener has been crushed |
| 5 | Measure the temperature again under load and record the result | Whether the temperature has returned to a normal distribution |
Why tightening alone is not enough
A terminal that has already heated and deformed is still in a poor state after it has been tightened, because the source of the heat is still there. The right procedure is to replace the damaged terminal, check that the lug surface is intact, and replace fasteners that have been crushed, because they have lost their clamping force. The ZYD-100 is fitted with phase failure and phase sequence protection, which catches part of these conditions, but resetting after the protection has acted without solving the contact problem only brings the same fault back.
There is another feature of heating at a terminal that is easily overlooked: it is tied to the load. On a lightly loaded machine the temperature rise caused by the contact resistance may not be obvious at all, and the fault only appears once the load comes up, which means a check carried out during a low load period can be too optimistic.
Put the electrical check into the routine inspection
The state of the terminals can be part of the routine inspection and does not have to wait for a trip. The method is simple: measure the three phase temperatures and the running current each time the machine is inspected, and write the result down. A change in the distribution between the phases says more than the absolute temperature of any one phase, and it costs far less than investigating after the protection has acted.
A loose terminal does not trip the same day, it only pushes the contact resistance a little higher every day.
