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I've come across a faulty kiln (pottery oven) and since the wiring diagram seemed simple enough I decided to try to find the fault myself. However I'm dumbstruck by a strange behavior of a lamp.

The oven is a powered by three phase power (but only use two phases, as evident in the diagram.) The two heating elements are controlled by a digital controller (the box to the right in the diagram) with a 7-pin connector. The controller has a switch that breaks the power (the L1 line) to the controller itself.

Wiring iagram

Looking at this diagram, it seems obvious to me that the lamp should light up whenever you connect the oven to the power outlet (given that the fuse "F0" is intact of course, which it is.) I mean, the lamp is in parallell with the controller so the voltage across it should be constant. However, in reality the lamp behaves like this:

  1. When the controller is disconnected, the lamp only flickers very dimly.
  2. When the controller is connected, and the switch is closed, the lamp lights up at full power. When the switch is open, the lamp is off completely.

What is going on here, shouldn't the voltage across the lamp be constant?

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  • \$\begingroup\$ Have you verified this with e.g. a multimeter? I would not trust such high level diagrams \$\endgroup\$ – PlasmaHH Jul 6 '18 at 11:51
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    \$\begingroup\$ Is your neutral wire intact? \$\endgroup\$ – Andy aka Jul 6 '18 at 11:56
  • \$\begingroup\$ @Andyaka I have only tested the neutral wire by disconnecting it at the upper left corner which left the lamp completely off, as expected. Did you have any specific node in mind where i should check the neutral wire? \$\endgroup\$ – Casey Tatum Jul 6 '18 at 12:28
  • \$\begingroup\$ @PlasmaHH I don't have access to a multimeter right now unfortunately (on vacation). Looking at the actual wiring, the diagram seems correct. But I agree, I should verify it properly with a multimeter. \$\endgroup\$ – Casey Tatum Jul 6 '18 at 12:29
  • \$\begingroup\$ you said that the kiln is faulty .... so why do you expect the behaviour to be predictable? ..... how is the kiln failing? \$\endgroup\$ – jsotola Jul 6 '18 at 16:41
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I agree with others that your N connection is suspect. When K0 and K1 are off the lamp will pull the lamp 'neutral' towards L1 and Lampe will fail to light.

When the heaters are switched on the N wire will, if the heaters are balanced be pulled to mid-way between L1 and L2.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. If the true neutral connection is lost then N will move to 'N', the line to 'N' voltage will be reduced to 0.866 [sin(60)] times the line voltage. The lamp should appear a bit dim and the kiln underpowered.

All of this can be cleared up by measuring incoming voltages.

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  • \$\begingroup\$ Well, this was a bit embarassing... I got hold of a multimeter and was a bit "disappointed" that all neutral wires in the kiln was intact. However, I also checked the extension cord that i use and the neutral seemed broke. I opened it up and there was no neutral wire at all! Being an computer engineer student I've only bothered to study single phase power, so it didn't really occur to me that cords without a neutral wire could even be useful. Let alone be out on the market. \$\endgroup\$ – Casey Tatum Jul 7 '18 at 22:29
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    \$\begingroup\$ For balanced loads such as 3-phase motors or heaters the neutral connection is not required. Where I work we always use 3P+E connectors if the cable has no neutral and 3P+N+E if there is a neutral. This makes the leads mistake proof and prevents the problem you have had. Thank you for accepting my answer. \$\endgroup\$ – Transistor Jul 7 '18 at 22:41
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Don't have enough rep or I'd just comment, but when you get back and have a multimeter, I'd check and see if there is any connection between pin 5 on the controller and pin 2. It looks to me that the wire connecting the lamp to L1 or N may be damaged in some way and it is finding a path through the controller (intended or not).

Also, what kind of problems are you having with the kiln? Or is it just the lamp malfunctioning?

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  • \$\begingroup\$ That's a good idea! The problem I have is that the digital controller is not responding at all (except for the physical switch I mentioned in the question.) The controller is supposed to indicate when there's a problem with the heating elements or the thermometer, but right now it seems completely dead. \$\endgroup\$ – Casey Tatum Jul 6 '18 at 13:32
  • \$\begingroup\$ Yeah, sounds like something went wrong and AC went through the controller. It'd be worth it to check out in my opinion. Luckily there aren't that many nodes, you may want to double check the wiring diagram and make sure everything is doing what it's supposed to. \$\endgroup\$ – Stiddily Jul 6 '18 at 19:15

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