Portable Electric Grill Shorting?

[b1177]

Hi,

I recently purchased a Philips HD 4433 portable electric grill/bbq for use in my apartment. I bought it second hand, but it was new and sealed in the box at the time. I took it home, set it up and plugged it in and it worked fine. I turned it off, and about 2 hours later came back to turn it on again. It worked well again, but this time after about 10 minutes the circuit breaker blew. I reset it, but subsequently, every single time I have plugged it in (regardless of which electrical socket I use) it will blow the fuse, this time without any delay.

I was wondering if anyone had any suggestions for reasons it might be doing this, as well as (hopefully) ways to fix it.

I am in Australia, and the appliance is Australian (220-240V, 50-60Hz). I would be happy to attach pictures, pages from the manual, etc, if it would help.

UPDATE: I left it to cool for about an hour, and tried to turn it on again. This time it stayed on for about 20 seconds before tripping the breaker.

b1177

 

[duke37]

Which breaker blew? Did you get over current or earth leakage?
Hot plates and some heaters have ceramic insulation which can get damp over time so that there is a leakage to earth. The leakage increases with temperature. I have a old cooker top in my shed with four solid hot plates, only one of which I can use to warm up my wood glue since the others activate the earth trip after a little while.

 

[b1177]

Thanks for the quick reply, duke;

It tripped the 30mA Residual-current device breaker.

As I mentioned before I had bought it sealed, but it was from a garage sale. According to the previous owner they had received it back in 1993, but never opened it. Could nearly 2 decades in a box have caused some degradation of the electronic components? If so, could it be causing this?

Edit: After reading your response I looked a bit more into the possible dampening of the heating element, and ran into this from wiki:

"Another cause of nuisance tripping is due to accumulated or burden currents caused by items with lowered insulation resistance. This may occur due to older equipment, or equipment with heating elements, or even wiring in buildings in the tropics where prolonged damp and rain conditions can cause the insulation resistance to lower due to moisture tracking. If there is a 30 mA protective device in use and there is a 10mA burden from various sources then the unit will trip at 20 mA. The individual items may each be electrically safe but a large number of small burden currents accumulates and reduces the tripping level. This was more a problem in past installations where multiple circuits were protected by a single ELCB.

Heating elements of the tubular form are filled with a very fine powder that can absorb moisture if the element has not be used for some time. In the tropics, this may occur, for example if a clothes drier has not been used for a year or a large water boiler used for coffee etc. has been in storage. In such cases, if the unit is allowed to power up without RCD protection then it will normally dry out and successfully pass inspection. This type of problem can be seen even with brand new equipment."

Do you think this might be the case? If I can power it up for long enough might it fix the problem? If that is the case, how might I be able to do that?

b1177

 

[duke37]

Seems to be the same thing as I have.

The obvious way of powering this is by an isolating transformer, however, the case will be live and I would not know where to find a sufficiently powerful transformer.
Site transformers (110V centre tapped) could perhaps do the job but I think the centre tap is connected to earth so this may have to be disconnected.The power would be down by a factor of four but it should still get quite hot.

You could try reversing line and neutral in case the leakage is at the line end of the element but I would not expect this to help.

Any modification to the earth system adds risk, do not make changes unless you know what you are doing. You could kill yourself or the cat.

 

[daddles]

Interesting thread; this is something I'd never heard of, so you learn something new every day. I believe that the usual insulator used in heating elements is MgO (magnesium oxide) and it's hygroscopic -- enough so that it's also used as a desiccant. But the water can be driven off by heating, so the advice of running it at heat appears to be good.

If it's blowing your ground fault breaker (what we call those thingies in the US), then that means there's current leakage to ground. The "obvious" thing for an experienced electrical person to do is to disconnect the ground to eliminate the leakage. However, this removes an important safety protection and should only be done by someone who knows what he's doing, as an electrical fault in the appliance could electrocute someone. The isolation transformer is a good idea, but the OP is probably unlikely to have one (I have a quite heavy 1 kVA one and I'd imagine it's still not large enough for this load). But I'd imagine that running the thing at heat for an hour or so should drive off the accumulated moisture in the heating element and it should then function normally.

If it were me, I'd make up a temporary extension cord with the ground disconnected, then set the grill outside away from anyone (and obviously not in the rain ), then power it on by plugging the cord into a socket. I'd keep everyone away from it while power was applied. Then, after I felt sufficient time had elapsed, I'd turn the power off and take the thing back to an outlet with GFI protection and test it again. If it didn't work, I'd repeat the treatment -- but only once. If that still didn't fix it, then I'd get out my troubleshooting tools and start looking to see if I could find the fault with (unpowered) measurements. If I couldn't find the problem in 10 or 15 minutes, the whole thing would be junked, as it's not worth more effort. Oh, and reconnect the ground in the extension cord.

 

[duke37]

I believe that Pyrotenax make cable with copper wires inside a copper sheath. The wires are insulated with magnesium oxide (magnesia) and the cable ends need to be sealed to prevent moisture getting in. The cable will withstand high temperatures so is used in critical applications.
I do not know how they could possibly make such cable.

 

[b1177]

Thanks again for the replies!

I have a friend who might have a transformer, but it will take me a few days to verify that. In the mean time I have opened it up and tried to check for any shorts. It doesn't seem like a complicated setup at all, and I couldn't locate any shorts.

In terms of creating a temporary extension cord with the ground disconnected, how would you suggest I go about doing that?

Here are a couple pictures of the device and the circuity:

http://i.imgur.com/33hXu.jpg
http://i.imgur.com/7pS1u.jpg

b1177

 

[daddles]

Since you're in Australia, I'll let the guys familiar with your power chime in, as I might tell you something wrong. However, the basic physics don't change, so this is what I'd do on either a 120 VAC or 240 VAC circuit here in the US. I'd just get an extension cord and temporarily disconnect the ground wire. Back in the 60's was when we switched from 2 wire to 3 wire outlets, but there are still lots of the adapters for outlets that only have two wires, so for me, I don't even need to mess with the extension cord -- I'd just use the adapter and make sure the adapter's ground wire didn't get connected to ground.

Again, by disconnecting the ground, you remove an important safety protection, so make sure the thing is operated where no one can come near to it. Run it at heat for an appropriate time, then try it on the GFI-protected circuit. If it doesn't trip the breaker, you probably have a fix. Then reconnect the ground in the extension cord.

Edit: you Australian guys might chime in if you have similar adapters available. In the states, you can get one for a buck or two at the local hardware store.

 

[duke37]

You do not need to open it up to check for leaks. Take a resistance measurement between the earth pin of the plug and one of the current carrying pins. There should be no connection, if so, heat it up and try again when hot. But you know this already because the earth trip goes.

 

[b1177]

I did that before I opened it up. It is 2100W and I got a resistance of 24.4ohm both before I plugged it in, and after I plugged it in, which sounds normal I believe?

b1177

 

[duke37]

24.4R is the resistance of the element and is correct. Your problem is the leakage between the element and ground. Measure the resistance between the earth pin and one of the element pins, it should be thousands of ohms.

 

[daddles]

duke37 is right; in fact, if it's blowing your RCD breaker at 30 mA, the DC resistance should be around 240/0.03 or 8 kohms.

 

[davenn]

24.4R is the resistance of the element and is correct. Your problem is the leakage between the element and ground. Measure the resistance between the earth pin and one of the element pins, it should be thousands of ohms.

and just to expand on that......

shouldnt really be tested with an Ohmmeter as it will not tell you if the insulation is breaking down under voltage. It needs to be resistance tested with a megger capable of 500V else the test isnt really valid.

The Ohmmeter will just show if there is an obvious short cct or at least a very low resistance path. What I'm trying to say is that you may get a false sense of security if it tests ok with an Ohmmeter, when it really needs hi voltage to break down.

cheers
Dave

 

[b1177]

Sorry for the long delay - I managed to grab a 2 prong adapter for a couple $ and attached it to the extension cord. I plugged it into the grill and turned it on, and it worked perfectly. Left it on for a good 15 minutes then turned it off. After that I tried plugging it in normally again, and as soon as I turned it on it blew the fuse. So at least it has been narrowed down to the ground.

I did what you suggested and measured between the ground pin and one of the element pins. I set the multimeter to the resistance setting of '2000' and it gives me a reading of between 1550-1950R - however it only gives me that reading for the first second I place the leads on the prongs, after that the multimeter just displays '1', and occasionally gives another high reading. Is that important?

What should my next move be?

b1177

 

[duke37]

1. Try reversing the power connections, they are Lucar connectors and you have plenty of wire to swap them over. This is somewhat of a forlone hope.
2. Heat it up as before for a longer time.
3. Get a new element.or take it to the tip.

I think you are on a loser here.

 

[daddles]

I'd run it for a significant period of time -- perhaps a few hours. If the ground fault is caused by something that can be cured by heating, that would likely fix it. Otherwise, unless you can replace the element, it's trash and will have to be thrown out. Do not consider operating it with the two prong adapter for normal use because you're disabling an important safety protection.

 

[b1177]

After several tests yesterday I think the unfortunate truth is that it needs to be junked. I am now fairly sure that someone along the heating element the ceramic core has broken, and then when it is heated the inner heating tube expands and makes the whole thing live which then trips it.

Thank you all for your help, it was really appreciated. At least I learned a lot

b1177