Terminal Burn Outs

[jpjamo]

Hi Guys,

I am a mechanical design engineer at a company that manufacture commercial filter coffee brewing equipment and hot water boilers.
I apologise in advance for a 'not well posed problem' due to my lack of EE background but here goes.

We have been seeing some burnouts between the heater element tabs and the flat 'quick connect' terminal that is attached to the element tab.
The elements we use are nearly all 2.8kW with supply voltage at 230V.
The element tab material can be either stainless steel or nickel silver.
As far as I read, these materials are both rated for fairly high temps - in the region of 200degC +.
Our element supplier has confirmed that the steady state temp of a tab when the element is energised tops out at about 90degC.
The crimp terminal we use are of the flat quick connect type. Apparently in years gone by we previously used elements with screw terminals and wires with crimped pin terminals.
We changed from this some years ago to the 'quick connects' to improve assembly/serviceability and also to be inline with our competitors.

It now seems that the 'quick connect' crimps may be the source of our problems though. I have taken temp measurements on the Neutral side of the element (which seems to have as much issues as the live side) and if the crimp is in any way loose or 'jiggled' around it is possible to get temps quickly up to 150degC at the connection – obviously because of increased resistance seen at the connection.
One other change that was made a number of years ago was to add in an 'insulating sheath' (basically a 50mm long piece of 12.5mm ID silicone tube). This tube covers from the thread section of the element right over and past the terminal connection - see photo.

http://files.engineering.com/getfil...5-bdfa-832f0f46abcf&file=Element_Set_up_1.JPG

http://files.engineering.com/getfil...3-9731-8827ec35aede&file=Element_Set_up_2.JPG

http://files.engineering.com/getfil...2e-f3bb4a1da38c&file=Element_Tab_Close_up.JPG

The temp at the connection can get hot enough to burn through the silicone tube – or turn it into a crumbly white mess.

http://files.engineering.com/getfil...a-4ccc-86f3-18457e6f86a8&file=Burnout_pic.JPG

This tube is meant to give more protection to service engineers if they took the machine apart without disconnecting first, but I am also wondering if this is contributing to our issues?

So my main query though is - are these 'flat quick connect' type of terminals reliable at all?
Should we just go back to the 'belts & braces' solution and use the pin terminals with screw connections on the element?
These quick connect terminals seem quite prolific amongst out competitors and other commercial cooking equipment though, so I figure they should be reliable enough!!? Are there different material types for these terminals that are better than others?


Any thoughts/comments/requests for more info or photos will be greatly welcomed.
James

http://files.engineering.com/getfil...b-8513-15505044539c&file=Pins_with_screws.JPG

 

[desertfox]

Hi jpjamo

I would think that placing the plastic tubes over the joint will hamper the heat flow to the surrounding air compared with a joint without the tube.
Can you do some tests on the temperature of the joint without the plastic insulating tube and compare that to a joint with the insulating tube fitted.
Your measurements of 150C doesn't say whether insulated tube was fitted or not.
I agree if your joint was more secure ie no movement of contact faces then you probably would achieve a lower joint resistance than one were you can have relative movement between contact faces. That said I assume that the connectors you are currently using are to an electrical standard and so the clearance between male and female connectors will fall within a tolerance band laid down by the standard.

regards

desertfox

 

[waross]

I suspect your purchasing department is buying the connectors from the lowest bidder. There is a difference in quality in this type of connector. Manufacturing tolerances are critical to the contact pressure. Are your element tabs clean when the connections are assembled?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jpjamo]

Hi Desertfox,

yeah the temp was taken without the silicone tube in place. Was just easier to jiggle the wire and see what was happening with it not in the way.

Though I do also have a query as to the best way to take temps off a terminal with frying myself or the equipment and not affecting the measurement by increasing resistence?? Is there a standard way of doing this?

Yeah the crimps are made to a standard which I think is IEC 70760 - flat quick connect terminations. I think there is an equivalent UL 310 for the US.
This basically gives tolerances on the tab (in this case it is 0.8mm +/- 0.03 - this may be a bit rough but it is something like that) and a similar one for the crimp.

The crimps though aren't the strongest things in the world and can be deformed, bent etc during assembly or service so I don't know how long they stay 'in tolerance' after they leave the supplier.

Thanks
James

 

[waross]

Any spade connection that I can "jiggle" is a connection that I expect will fail. The differences in quality:
1, It is more difficult to twist a good quality connector.
2, The small gaps that provide the clamping action are more uniform in a quality connector.
3, The gaps are more prone to spread and reduce contact pressure in poor quality connectors.
I have difficulties purchasing good quality replacement connectors in Central America. I often have to squeeze the connectors with pliers to achieve adequate contact pressure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[desertfox]

Hi jpjamo

Can you take some temp measurements with the insulator tube fitted.
I can only relate to my time in switchgear when we used to put thermocouples onto the busbar joints then apply a current and leave untill the temperature didn't increase anymore, all I can remember that it was an IEC spec but which one I don't know, however I am sure that there must be similar tests for your appliances which with a bit of digging you may find. I assume the measurements you took were with a thermocouple or was it just a probe you placed on the joint if so the thermocouple maybe what you need.
Don't know whether its feasible but can you measure the resistance of the new joints compared with the old style joints to get a comparison?

regards

desertfox

 

[Mike2468]

As stated, there is a massive difference in quality between Fast-on (female) connectors. Stamped brass vs phos bronze, good supplier vs other, etc. In addition, you have a thermal connection that is constantly exposing the junction to thermal stress. If the junction is loose, you will have a failure -- guaranteed.

You might want to contact a major supplier such as Amp/Tyco for their comments.

 

[jpjamo]

Thanks all for the replies.

Those subtle differences in quality clearly make a big difference to the outcome.
Just one thing on the suplier quality - we manufacture most of our Equipment in house still but some of our 'lower end' stuff is made in the far east. The interesting thing is that we use Utilux crimps here and our far east supplier something else (This something else is still TBD!) Anyway, we have seen the issues on Utilux crimps. A collegue (since left) showed me one getting, litteraly, red hot last year on an element - unfortunately I was not in any way involved with this issue back then.
We have improved checking processes in house to inspect the connection and we measure the temp of the wire just after the tab to make sure it is below 55degC - the inference being that if the tab/connector are at 90degC then the wire a short distance down should be below 55. We use a 'clamp' type thermocouple to take the wire temp. This again was implemented before my time.

I do use a thermocouple wire to take the tab readings.

I guess what I am wondering though is the general consensus that Fast-on type of connectors are worth sticking with for this specific application? Sure we can still use them on all the other connections but for the element, is it better to go to pin terminals and screws?



Thanks
James

 

[Mike2468]

I would recommend a firmer connection such as a screw terminal. A slide-on connection that is exposed to high heat and amperage will only get hotter over time, especially after you use it a few times (such as after heater replacement).

 

[mcgyvr]

I have had no issues with quality (AMP/Tyco) "faston" type connections (.250" tabs) carrying 25 Amps constantly.

With a proper tab and proper connector there should be no issues.

Do a test where you take a few of the quick connect terminals straight out of the box and test them versus some made by another manufacture. (Even your local hardware or electrical supply house will have better quality ones)

I believe there are 2 different thicknesses for the male tabs too (.020" and .032") that will make a difference also

 

[itsmoked]

You really just need to spec a specific excellent quality female spade, then buy only that company's spade. You cannot cover the spade with anything that hinders air flow. You do not even want an insulated crimp. It is imperative that convection can occur around the connection. This would be the correct application. Anything else like covering tubes is a mis-application.

I would also suggest a force meter tool so the assembler sees the actual force applied as the spades are installed. If there is enough force then there will be a gas tight connection between the two connector halves. This will prevent the oxidation that will otherwise occur. This oxidation will lead to increased connection resistance. This means power loss at the connector, which means heating. That is always the end game on these connections.

Yes spades are fine. I would not use slow, problematic screw terminals. You just need to use good spades properly and they will outlast the elements.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jpjamo]

Hi again,

well I have definitely learnt a few things so thanks to all.

The whole air flow requirement does make complete sense when you hear it. I guess in our zealousness to protect the assembly and servicing guys we shot ourselves in the foot a bit.

Keith - I like your arguments for sticking with fast-on spades but I hadn't thought 3 turns of a screw terminal was slow & problematic .

One of the other funny things is that if we were to go to screw terminals our current 'protection method' using the silicone tubing wouldn't work anyway due to the orientation of the pin terminal.

I was a little suprised that even just insulating the crimp is not recomended though as these are fairly standard it seemed??

Anyway I will get onto to Tyco and see what more I can learn.

Thanks heaps again.

James

 

[itsmoked]

Hi James.

Normally they are insulated but in the special case of connecting to heating elements you need the copper wire to wick away heat and the convection. That whole zone is a temperature transition area where the heat conducting down from the heating element must be shed or the terminal, or even screw terminals will oxidize into oblivion.

If you really want to protect that end you need some little ventilated plastic cage to snap over it. One that lets air thru.

Good luck.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jpjamo]

Hi again all,

brief update and more questions!

I have been on to Tyco trying to get recommendations but as it's holiday season over here it will probably be a week or more before I get some good feedback.

Anyhow I have been checking the element tabs a little more in detail and one thing that concerns me is the 'flatness'. IEC 60760 specifies that the thickness of the tab should be between 0.77mm and 0.84mm (for a 6.3mm tab).
It then gives a definition that "all portions of the tabs shown shall be flat and free of burs or raised plateaus, except that there may be a raised plateau over the stock thickness of 0.025mm per side, in an area defined by a line surrounding the detent and distant from it by1.3mm"
Wow - sometimes I love reading standards.
Anyway, surrounding of a circular hole of 1.7mm diam would give a diameter of 4.3mm for which there can be a plateau of 0.025mm.

This I presume is to allow for the effect of punching a hole (or dimple) in the tab which will give a raised area around the hole.
What I have measured using a micrometer on our ‘double-tabs’ is that the plateau around the hole (which is actually more of a curve over the width of the tab), is about 0.08mm in height. The thickness of the tab is 0.79mm close to the sides but if you measure across the width (by bending one side out of the way) it is about 0.87mm.
Please note when bending one half out of the way I did not ‘grab’ the other half!

http://files.engineering.com/getfil...5dc-4ff3-a8c7-19bb09206e48&file=Tab_photo.JPG

Does this sound like it would be a problem that could be causing poor contact and hence high temps??


The other thing I have noticed is that Tyco have a crimp inspection guide which states that the stripped wire should protrude no more than 0.8mm past the end of the folded part of the crimp.
The main reason it seems for this recommendation is that the wire could interfere with the tab at insertion.
While I don’t think this is causing us a problem from an insertion point of view, would it be likely to cause any other issues? The discolouring/oxidation shown in the photo does seem a little strange.

http://files.engineering.com/getfil...28-8694-d719c17e063f&file=Wire_overhang_1.JPG

http://files.engineering.com/getfil...09-a6d8-2fa3512188e9&file=Wire_overhang_2.JPG

http://files.engineering.com/getfil...75-8542-9d5210bf50eb&file=Wire_overhang_3.JPG

In the meantime I have also been testing some screw terminal element samples and it has to be said they are performing fairly well. Almost an immeasurable milli-volt drop across the connection and very low temps in comparison. Also helped by the much greater surface area of the screw terminal to help things cool down quicker.

Thanks again
James

 

[mcgyvr]

I would not attach that double tab to the screw like that. That just looks like an issue waiting to happen.
Why not just use 2 nuts and a ring terminal to fit right over that screw stud from the elements? Screw on 1 nut first, then ring terminal then another nut and be done with it.

 

[jpjamo]

Hi Mcgyvr,

thanks for your suggestion.
The threaded rod wasn't always threaded in the past as far as I can work out. A previous supplier just welded to plain rod. Our newer supplier welds to the threaded rod instead as this is their standard method. So far there have been no issues with this joint but either way you're idea is a good one and worth investigating.

One concern about such a fixing method in the past is that you are applying torque to the rod which is attached to the heater element coil. It was suggested that if you were to over torque the nut then you could snap the fairly fragile weld between the rod and the element coil. Something I can't prove or disprove at this point.


Thanks again
James

 

[waross]

A couple of observations;
1> I have found quality problems with the female connector on the end of the wire, not the flat tab.
2> I wonder about the ampacity of the weld to the threads. Does the welding operation deform the threads enough to produce the same contact area as a weld to a straight rod?
3> Can you extend the rod so there is about 3/16 of an inch between the porcelain and the spade connector? This will allow some of the conducted heat from the heating element to escape so that the connection tab may run cooler. Alternately, the tab may be extended to allow a heat radiation area.
4> It was not clear from the photos, but given the nature of the tabs, two flats, I hope that you are not using these as splice points for daisy chaining. When you daisy chain on such a joint, you have a second current and a second contact point developing heat that ads to the heat from the working connection and the heat from the element. In addition, if you double the current (I) across the contact resistance (R) your i^2R may become problematic. If you must make feed past joints, do it on a cold connection.
5> Given the millions of these connections working well in appliances, and the manufacturing economy and ease of Q/A with spade connectors I would suggest finding and fixing the problem rather than changing to a threaded nut based connection. That is very good for one fix in the shop or on a service call. In a manufacturing environment, there are cost and Q/A disadvantages.

Bill
--------------------
"Why not the best?"
Jimmy Carter