Help with Hobbytech 40

[roobarb]

Hello

I have been using a Hobbytech 40 kiln for a few years, I think early 90s age but it could be older. The label has burnt off. I run it off a 13amp supply.

The kiln isn’t in the best of condition. I am planning on buying a larger, more suitable kiln in the coming weeks but until then, I have to make do and mend.

Over the last few months, the wires are burning out and firing is becoming tricky. I have to replace the wires ever 4-5 firings. The usual wire to go if the one at the top element connection. Anybody got any idea what would be causing this? I have removed and repacked the insulation as I could see some heat could be escaping, but the problem continues to happen.
 

As you can see from photo, wires on right hand side to the lid safety switch are also now damaged from heat. Are these straightforward to replace? 

I was considering having a kiln engineer come to look at this, but the quote for the service alone without parts was over £350 (I only paid £300 for the kiln several years ago, and I’ve got more than £300 worth of work out of it).

Any help from a knowledgeable person would be appreciated. Thanks.

[neilestrick]

You probably have a dirty connection at the element. Replace the nut and bolt, washer, etc, clean off the end of the element with a wire brush and a little WD-40. Replace the wire, too. Don't just cut it back. You need everything clean and shiny. Any wires that are looking crispy should be replaced.

[Bill Kielb]

16 hours ago, roobarb said:

Any help from a knowledgeable person would be appreciated. Thanks.

Cleanliness of the connection is definitely key, but those wires appear as building grade and not high temperature. Any heat  escaping or heating the connections is definitely a contributor that always needs to be dealt with as well. Replace the wire with the appropriate high temp. rated wire.  The existing thermoplastic insulation is burning and likely causing your carbon staining. High temp. connections rarely have thermoplastic covering on them.

New proper temperature rated wire, replace the existing element connection new bolt, nuts, washer.and burnish all bright with fine sandpaper if necessary. If your bolt is slightly long, no worries it will serve as a heatsink as long as it does not create a risk of electrical short. This will actually sink heat away from the wire which helps the connection last longer.

Don’t worry about reasonably over sizing wire. If your location requires 1.5mm wire for 20 amp appliance use, using  2.5 mms wire rated at 27 Amps  will last longer and be a better heat sink. Finally all connections nice and tight. Loose connections heat up, waste power and heat up some more till they burn up.

If this is high temp wire, double check it’s amperage capability. It does look like it might have been replaced at some point so I would make sure it is heavy enough to handle the current plus a reasonable factor for safety as wires operating in a heated environment generally are derated by operating temperature.

[neilestrick]

6 hours ago, Bill Kielb said:

but those wires appear as building grade and not high temperature.

Good catch! I was posting from my phone and didn't notice.

[roobarb]

Thanks for the advice on this. I have removed nut and bolt today with a view to replace completely , and I have ordered a replacement porcelain insulator because the current one was damaged at the reverse.

The wire was purchased directly from Cromartie and is advertised as high temp (https://www.cromartiehobbycraft.co.uk/Catalogue/Ceramic-Kilns-Electric-Kilns-Pottery-Kilns/Kiln-Spares-Kiln-Elements/Cable/Heat-Resistant-Cable-25mm-Red-HRC-RED-25) but since asking the question on here and spending a bit of time on google, I’ve found that there is high temp wire available elsewhere, wrapped in a fibreglass material, so I will order some of that instead and replace all wires that are damaged or on the cusp.

Incidentally, the metal at the top of the kiln where the bolt and insulator sits is in a bad way, with lots of corrosion. I will attempt to clean this up as best I can, as you have both suggested for the element connection.

Thanks for your help.

[Bill Kielb]

3 hours ago, roobarb said:

I’ve found that there is high temp wire available elsewhere, wrapped in a fibreglass material

Just some thoughts here
High temp wire insulation,  the basic types would be MG (Mica Glass) rated to 450c and TGGT (Teflon, glass,glass, Teflon) rated to 250c with MG being the most expensive. (There are slight variants of both as well but these are the most common) Since this is already rated wire from Cromartie then we can assume it’s sized right for the load and the insulation should last. So next step is to make sure the hot parts of the kiln are well insulated and any ventilation designed into the control box is performing. Often vent holes punched in the bottom of the control box and some vents punched in the top to take advantage of the rising currents of air. These must be open and as free as practical.

One caveat, these openings depend on the room air being relatively cool so if the kiln is in a small space that heats up excessively then the ventilation  will not perform as well as designed. The high degree of corrosion in your picture could be an indication this room gets pretty warm. It’s just a picture, so could is the operative word here.

So knowing the wire is likely right then my next best thoughts: 1- make sure the kiln is as well insulated as practical from the controls and wiring (Tight packed insulation reduces its value, well filled, no voids and loose is best)  and  2- ensure any openings used for control box ventilation are open and free. The vents I see punched in the bottom  should have as much space as practical between them and the floor so air can rise up through them and hopefully freely exit out some prepunched vents already in or near the top of this enclosure.

Keep the room as cool as practical and keep any wiring as far away from the kiln shell  as practical.