A very common cause of electrical equipment failure is relay or contactor malfunction. Relays can stop performing as intended due to burnt contacts, dirt, electrical arcing, but they can often be easily repaired by following the steps below.
This procedure doesn’t apply to solid state relays and solid state contactors, since those relays don’t have moving parts and are manufactured as a complete sealed unit that can’t be taken apart. But for most mechanical relays, where the contacts can be accessed by removing a plastic cover and a few parts, repair is possible, and relatively easy.
The industrial relay in figure 1 will be used as an example—most relays will look similar once they're opened up. Popping the cover is just a matter of finding the screws or tabs.
Figure 1 shows a more recent Schneider relay with its plastic parts held together with little tabs, while in Figure 2, the relay is held together with screws, one of which is indicated by a red arrow. But both are practically twins once you get inside. The frame will have multiple contacts built into it (how many depends on the relay’s purpose), a coil, and a plunger style armature resting onto a large spring, the purpose of which is to return the armature to its normal position while de-activated.
For each pair of contacts on the frame, there's a contact strip on the armature that will connect or disconnect them.
Finding Bad Contacts
Step 1 - Check with Multimeter
The first thing to do is determine which contacts are defective. Figures 3a and 3b show the relay with red arrows pointing at the armature, which is the blue part shown more clearly in Fig. 3b with the covers removed. Fig. 3b also shows six sets of wire terminal screws at the bottom of the picture, which is paired to corresponding screw terminals directly opposite on the other side of the relay (on top of the picture).
Place the probes of a multimeter set on continuity or Ohms across one pair (set) of terminal screws while measuring resistance—once with the armature in normal position (up), and again while depressing the plunger all the way down. Compare the readings, which should show open circuit in one case and closed circuit in the other. Any of the six sets of contacts that show open or closed circuit in both cases is faulty. Those bad contacts should be identified and noted.
Step 2 - Repeat For Each Set of Contacts
Repeat the process for each set of contacts. If the readings are all good for all of the sets of contacts, the cause of the malfunction is probably some loose dirt or dust causing an intermittent connection issue.
Opening up the Relay
Examine the relay to get a clear sense of how the cover was installed over the frame, and how many pieces it has. If there are no obvious screws holding together the assembly, look for little plastic tabs around the parts of the cover (see red arrows in Figures 4 and 5).
Step 3 - Gently Pull the Tabs
Use a small flat screwdriver (as shown in Figure 6 below) to pull lightly at the tabs—the parts should come easily away from each other, exposing the interior frame. It’s very important to be careful not to over-stretch or break any of the plastic tabs. Figure 4 clearly shows the cover (at the bottom of the picture) taken away from the frame (at the top of the picture), and exposing the screw terminals and the armature.
Taking the Frame Apart
Step 4 - Unhook the Frame
What really goes on inside a relay is quite obvious once the frame is taken apart. In this case, there is one tab on each side of the frame that needs to be unhooked (Figure 6). Once this is done, the frame will come apart in two pieces, revealing the armature at its core (the blue part in Fig. 7). The contact points should now be completely exposed and ready for maintenance.
Cleaning the Contacts and Insides
Now that the points are fully exposed, they should be given a thorough examination for corrosion, pitting, dirt or other anomalies. If the contractor was used in a woodworking environment, for instance, there could be tiny sawdust particles floating around. If small debris issues are the root of the problem, and the points aren't damaged, all you'll need to do is clean this out.
If the contacts show any damage, however, the procedure must be taken a step further. Remember to never burnish or file relay contacts, never use grease and other coatings that could collect dirt or dust, and only clean with a well known brand of electronic contact cleaner—low or medium current contacts often have a very light plating of gold or silver applied to them that could be easily come off without proper care.
Step 5 - Apply Cleaning Solution
The two parts of the frame and the armature must be handled with extreme care and attention, because they’re made up of a lot of small parts that could easily fall or get lost. Delicate focus at this point is extremely important. A pair of latex gloves should be worn for this next step.
The first half of the frame can be picked up with one hand so the contacts are visible. With a contact cleaner aerosol in the other hand, proceed to thoroughly spray the contact points. These products are designed to clean contacts, and remove all traces of oxidation, without leaving any residue.
If professional contact cleaner isn’t available, vinegar is the best home substitute.
Step 6 - Brush the Contacts
Once you've applied the solution, take a small nylon brush (or toothbrush) and gently scrub the contacts clean.
Step 7 - Clean and Brush Frame
The other part of the frame can then be submitted to the same treatment (from steps 5 and 6).
Step 8 - Clean and Brush Armature
Take extra care with the armature since its contacts (points) are extremely flexible and could easily come off. Spray the armature with contact cleaner and scrub it delicately with the brush, using extreme care.
Step 9 - Reassemble Frame
After completing this final cleaning, reverse your disassembly to get the relay back together and working again.
Step 10 - Re-Check Contacts
Repeating steps 1 and 2 with the multimeter will tell you whether the process was successful. If so, the restored relay can be put back into action. Good luck!