Diagnosing a Slow Firing Manually Controlled Kiln & General Tactics

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  1. Look for the nameplate data. Plug the amperage and voltage labeled here into Ohm's Law to see what the resistance for the whole kiln should be. If the nameplate is missing you can email the factory to try and figure out what model it is. Measure the inside dimensions of the kiln, take whatever resistance readings you can, let us know whether it has Hi-Med-Low switches on infinite type switches and describe anything else you can about the kiln. An emailed digital picture can be very helpful.
  2. Measure the total resistance of the kiln. Unplug the kiln or turn off the power if you cannot unplug it when measuring resistance in these circuits. Now turn all switches to high, and turn the kiln-sitter on. Measure the ohms from the prongs on the main power cord–-from the two "hot" blades, not from the ground or neutral. If there is a reading, it should be within 9% of what was calculated with Ohm's Law. The resistance can only be lower than what the nameplate calculations would indicate if the wrong elements were installed in the kiln or the elements are so old that they are squashed into each corner all the way around the kiln. Look for overheated connection if low resistance continues for any length of time and replace elements immediately
  3. Measure the resistance of each branch circuit. Turn the switches OFF. The switches must be off or the meter will read all the branch circuits at once. Measure branch circuit resistance with the kiln power OFF from the two flat prongs (not the ground) of the plug-heads of each kiln section. On other kilns you want to determine how many elements are in each circuit and how the elements in each circuit connect together and to each circuit's power wires. Take the branch circuit resistance reading at the point where the power wires connect to the element(s).
  4. Determine series or parallel. Look to see if the elements are wired in series or in parallel with each other. Even in L&L's latest kilns you would still have to either take the element box off or look at the kiln's wiring diagram to determine this.
  5. Check individual element resistance. Try to get a single element's resistance reading by either calculating it if they are in parallel or by measuring it with the meter if they are in series. You may need to disconnect wires to isolate as much as possible of each element.
  6. Take a voltage reading in each branch circuit at either the element connection to the power wires or at the control box receptacles on later L&Ls. Measure the voltage at the main power supply. If there is a considerable voltage drop from the main power supply to the element connection to the power wires then there is a corrosion or connection problem. Badly corroded connections need to be replaced immediately. Both parts of the connection should be replaced at the same time. Check your plug and receptacle connections, especially the main power cord and receptacle.
  7. If the measured resistance is slightly more than 9% over the calculated resistance and this correlates with the problem (slow kiln), you should ideally replace all the elements, or at least those with readings that are too high. If you do not replace them all at once the kiln may heat unevenly (this is much less of problem with kilns having the zoned design with ungraded elements rather than with kilns that have graded elements.
  8. If all of the element resistances are fine but the resistance of the whole kiln is not, the problem must be in a branch circuit. 
  9. With the kiln on, run a voltage test on the receptacles or at the connections to each element in each branch circuit to see which is the bad one.
  10. With the power off, open the control panel and visually inspect the branch circuits. Check branch fuses if the kiln has them.
  11. Locate the two wires that begin the bad branch circuit from the bunch that come from L1 and L2 on the main power block.
  12. Follow those wires to where they connect to the first component in line, probably either a fuse block, a relay or a switch.
  13. With the power ON, and any kiln-sitters or switches on High (so that the elements would come on if they could), take a voltage reading at the point where these two wires connect to the first component in line. The reading normally should be the same as what it is at the main power block. If it is not, one of the wires between the main power block and the first component is bad and needs to be replaced.
  14. If there is voltage there then take another reading after the first component at the point where the two wires continue onto the next component or to the element connection. If there is voltage after the component then the component is working.
  15. To determine whether the contactor or the switch is bad, first follow the wires from the load side of the switch to the contactor. 
  16. With the power all on and the switch on high, take a voltage reading where the two wires from the switch to the contactor connect to the contactor. If these readings are the same, then the contactor is bad.
  17. If there is no voltage present, then follow those two wires back up to the load side of the switch and measure the voltage there. If the voltage readings are the same, then the contactor is bad.
  18. If there is no voltage present, then follow those two wires back up to the load side of the switch and measure the voltage there. If the voltage reading is the same, then one of the wires is bad.
  19. If there is no voltage present at the load side of the switch (power all on, switch on high, then be sure voltage is coming to the switch; if it is, then the switch is bad. Replace the switch and if the problem still persists then repeat the test; you will most likely have to replace the contactor as well.
  20. If there is no voltage after the first component in line and it is not a relay/contactor, then just replace it. If it is a fuse holder, just replace the fuse (usually a bad fuse means there is a short somewhere in the circuit). Use a "continuity" tester to test for bad fuses. Always check tightness of connections in a questionable circuit.
  21. If there is voltage after the first component then move along the circuit from the main power block towards the element connections, testing for voltage before and after every component until you isolate the problem. Voltage readings taken from between the elements (and from between resistors in general) give a reading that reflects voltage which is half the supply voltage with two elements in series, and either one-third or two-thirds the supply voltage with three elements in series (depending on which side of the middle element in the series the test lead is placed).