Official Luthiers Forum!

Hi All,

I checked with Lance before posting this and got his blessing, so please don't let this become a discussion of whether this kind of posting is relevant or not.

I just purchased a brand new set of side bending heater blankets (one long one and one short one for adding extra heat to cutaway bends) and found some safety issues with the wiring that I need to bring to everyone's attention.

I tested the smaller blanket first - plugged it into a normal AC outlet - and it worked just fine. When I plugged the large blanket in, there was instant arcing and a loud bang as one of my AC circuit breakers tripped. I reset the breaker and tried it on another circuit, with a higher current rating. The same thing happened, immediate arcing and the breaker tripped instantly.

I got my multimeter out and measured the two heating elements. The smaller one measured fine, but the larger one was a dead short. It would have to go back. However, being an inquisitive sort, I decided to open up the in-line AC socket to see if the problem was in there and perhaps fixable. Here's what I saw inside:



The wires were bare for about 1.25" and were crossed over and shorted!
The insulation wasn't just "pulled back a little", the wires were stripped right back like this. The wiring was obviously never done properly at the factory. It's a good job I had good circuit breakers - this kind of fault can start fires! When I separated the wires and measured the DC resistance of the heating element, it appeared to be fine, coming in just under 15 Ohms:



I cut back the orange outer insulation tube and remade the wire ends properly. I dispensed with the black rubbery cable entry grommet (it really isn't necessary - the wiring is actually safer without it, as it is held tightly so it can't twist or rotate) and reassembled the in-line socket safely, like this:



While I was at it, I decided to inspect the wiring on the inside of the small heater. The screws on both wires were loose (hard to show in a photo), and the outer orange insulation was already splitting on the black rubbery cable entry grommet, allowing it to freely rotate which, of course, will lead to eventual twisting and shorting of the wires inside the in-line socket:



After I remade the end on the small blanket heater, I tested them both again - this time they were fully working with no problems, and should be good for quite a few guitar side bendings to come!



In my opinion, EVERYBODY who has/uses these types of heating blankets should immediately do the following safety checks:

1. The internal wiring of the in-line socket should be inspected to make sure the wires are not over-stripped and potentially shorting (if so, the wiring should be re-made)
2. All screws should be checked for tightness
3. Check to see if the in-line socket can rotate on the end of the orange outer insulation. If it does, then you really need to remake the wiring/cable grip properly, without the black rubbery cable entry grommet in place.

If you are competent and can do this yourself, then, entirely at your own risk, have a go. However, if you are in the slightest bit hesitant about your abilities to re-wire things, please have a qualified electrician do this work for you.

I bring this to everyone's attention purely in the interest of public safety. I'm not going to say where I purchased the heaters. No finger pointing is necessary. I just want y'all to stay safe out there!

Regards,
Dave F.

_________________
Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"

Good job Dave buddy!

Many thanks for looking out after us and I am very glad that you did not get injured either.

Very usefull thread Dave!

I just checked mine and it looks ok, although I will apply the modification you did (getting rid of the grommet).

Thanks!

_________________
Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

indeed this is very useful information...on a tangentally related subject I had a mishap with a brand new router bit 2 days ago...I am making a whole bunch of curved 1 1/2" thick curved caps for half walls in a bowling alley..now, the curves are not true radiuses as said radius is about 50' and the framer of course had no 'real' way of making an exact match...hence the only way for me to make a cap to match is to use the existing half wall as a pattern....I decided to use 3/4" mdf 1x2 nailed on to the edge of the wall, shim it where required to make a smooth curve, join and glue 6' long pieces of 2x12 maple to the cap and then use a 2" long 1/2" fluted bit with a bearing on the tip to follow the pattern as established by the 1x2...after transferring an approximate pencil line to the top of the rough cap I used my Skil wormdrive to remove most all of the material so that I could make the final cut with the router (I am including this 'extra' information in case anybody finds it useful at some time)....I then proceed to chuck up a brand new bit (brand name deleted to not break any forum rules) and start the real work of making the finished cap...great plan eh????...well it turns out the screw holding the bearing on was not properly tightened!!!! OMFG !!!! said bearing came off and I feel very lucky to have 1: found the parts in all the shavings 2: reacted quickly enough to the bad scene that about 5 minutes of careful belt sanding and use of CA to lock the resultant damage of splitting and blowing apart of the maple 'fixed' the issue...

The_Point being that with today's apparent lack of quality control it is advisable to perhaps check out all new items before they are put into use!

I have ground fault circuit interrupters everywhere, as an added precaution.

Dave-
Thanks for the 'heads up'.
I just checked my blankets and they seem OK, though the terminal screws were not as tight as I like 'em.

Where did you buy your blankets?

John

Hi John, I will politely decline to say where I purchased them, since it's not that relevant. The distributor isn't the party that messed up here. The manufacturer is. These heating blankets are pretty much universal - they all come from the same source as far as I can tell, regardless of where you purchase them from.

I have sent an email to the distributor I bought them from to give them a heads up.

I sincerely hope that no one else finds any with serious problems, and that mine were an anomoly.

Cheers,
Dave F.

_________________
Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"

Actually, there are different sources (manufacturers) for heating blankets. I've bought from two sources and the blankets are quite different in construction.
Yours look similar to the ones I bought from LMII, but you may have gotten yours from another supplier.
I do think that a retail seller has some responsibility for the safety of the products they sell.

John

John, I agree with Dave here, there is no need to single out any vendor. The reason for Daves post was to
alert anyone that may have a heat blanket to take a look at them.

Determining responsibility can be left to someone else and some where else.

Thanks!
Lance

_________________
Support the OLF! Bookmark our STEWMAC link Today!
Lance@LuthiersForum.com

I disagree with the policy to not reveal suppliers of defective and/or dangerous merchandise. So what you are saying is that we need to buy our heat blankets and then take them apart to determine if the wiring is correct? What if we are not qualified to make that determination?

In the discussion of OLF policies, the comparison of other forums' policies is often brought up with the MIMF being the subject of much comparison, and often ridicule, for not allowing certain topics. Well the MIMF has always allowed product reviews with supplier names also allowed. After all, there is no defamation if the discussion sticks to facts, and not opinion.

Lance, I respectfully request you reconsider this policy. As it is, you are giving the appearance of placing commercial considerations above safety in this particular instance.

The explanation of the supplier not being responsible as its the manufacturer's fault is just avoiding the real issue.

I said no such thing. Barry, please dont put words in my mouth/


Good for them! The MIMF, its a great resource!


Policy? Barry, what policy are you talking about? I simply agreed with Dave that there is no need to single out any vendor. And I am intitled to that opinion thank you.


Lets not micro analyze this post and just be thankful that Dave pointed this out
It is Dave's decision to concentrate on the "problem" and not the manufacture. I can respect that and I would expect that everyone else would too.

Thanks Dave!

_________________
Support the OLF! Bookmark our STEWMAC link Today!
Lance@LuthiersForum.com

I know for a fact that the blanket was not a protherm asI wire all the blankets personally and test them so such a thing doesn't happen. SAFETY cannot be compromised as you can see what happens.
The posting I am sure is for the reason of safety . I to take acception to the statement that they are all the same. Blanket manufactures vary the process from maker to maker. I don't think anyone would want a bad unit to get out but this is why some blankets cost more than others.
I finally wore out my first prototype blanket and it was because a wire was bared. The blanket did work but it was a safety issue. I advise to allways check your heating equipment for bare wires and signs of abuse. This is something that will be covered in the OPEN HOUSE
Hope to see you all
john hall

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Look like I touched a nerve here. Sorry Lance. My intent was not to offend or condemn you. I was trying in my own clumsy way to encourage you to make the OLF an even better source of information to your participants. I take a very proactive stance when it comes to safety.

I'll go crawl back under my rock now.

Nope, no nerve.
I should have included a few 's and 's.
I can just smell a thread going south pretty far in advance these days and
this one had the making of one.

No offence taken Barry.

_________________
Support the OLF! Bookmark our STEWMAC link Today!
Lance@LuthiersForum.com

I don't think you can really be a guitar maker without also being a bit of a carpenter, plumber, and electrician. This stuff isn't rocket science, and there are a ton of books out there on how it all works. It really is quite simple because it all really makes sense if you trace it all out and pay attention to color codes, Amperage loads, voltages, and especially grounding.

If you really want safe benders, it would probably be a good idea to ground all steel slats used in the process. Do I? Nahhh, but I do know enough about electricity to keep an eye on everything.

I think the heads up is great. I also think that contacting the distributor was wise. Maybe all of these units should be tested by the distributor before they get packaged and go out. I don't think it's always enough to have the manufacturer test them. That wiring looked like the work of a disgruntled employee.

I had a weak solder joint on a blanket recently and took it back to the distributor even though I knew the warranty was up and it was replaced without any problem.

_________________
Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

My apologies to John Hall of Blues Creek Guitars for saying that the source of these blankets was common, thus implying that they all suffered from this problem. I should have spent more time researching that fact before posting.

So, it does appear that there are several different heating blanket manufacturers after all.

The ones John Hall wires up, tests and sells (Protherm) are not the subject of this thread.

However, no matter whose heating blankets you are using, it would pay to do a check every few months to make sure they are still safely wired up and not worn through to the heating element anywhere.

Rick - I realize that probably everyone on this forum can take care of the wiring themselves, but it would be remiss of me not to include the standard "if you're not 100% sure about what you're doing, contact a qualified electrician...." disclaimer bit in there to cover both myself and the forum owners from liability in case some idiot (<insert name of deceased here>) goes and electrocutes themselves as a result of reading this thread!

Cheers,
Dave F.

_________________
Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"