Am I Getting OLD??? Totally Confused

I assume these are elements that are sealed inside a metal sheath. Sometimes the inner nichrome wire moves enough to contact the sheath (which should be grounded) and you can get low insulation readings that come and go as the wire expands and shrinks due to the changing temperature, but it seems unusual for that to happen on a brand new unit. Of course the quality of the unit depends upon where it was made so check the label and insist on a new unit that has a name brand that you can pronounce.

From your original "cold" IR readings calculate the cumulative leakage current for all units. 300mA will never hold all four starting together added to which there will be the inherent leakage of the thyristor control.

Can you set the leakage trip current higher? Just to see what happens of course not a permanent setting. Maybe even measure the current in the grounding conductor.....

The wavering insulation resistance values are a bit weird.

Are you performing the insulation tests at the same test voltage setting on your instrument?

I have set the mAmp trip at 1 AMP.. The unit works well. my biggest problem is:-

1).I cannot operate this heater at 1 amp earth fault...

2).The varring insulation resistance when hot and when cold. The manufacture tells me I have to have above 2 M ohms at ambient temperature.. I think that we should have little or no difference between heating elements and GND at any time...

3) Manufacture is insisting that this is humidity in the elements ( which are sealed in a metal sheath)- I disagree.. the unit has operated 21 times for 110 mins each time with a setpoint of 280ºC surely any "humidity" would have dried up by now..

Yes I am using the same instrument to meassure and at the same voltage... To remove any doubt we had I got na external electrical contractor to also measure.. he got the same readings..

I'm with you on the mineral insulation inside the element must be dry by now. Unless it's sucking in water from the air when it cools but then you've observed that the insulation resistance is higher when cool...

Earth leakage on new elements is not uncommon and the cook out suggested by the manufacturer is normal practice.

If this is new kit and the manufacturer tells you what you should be measuring and the spec can't be achieved then it sounds like a warranty issue. Doesn't it?

To sum up... for the past 2 days we have been baking the heater elements.. (125ºC for 35 hrs. we sealed the terminal box switched off the panel fans and sealed the fan exits with duct tape... Purged the terminal box with dry Nitrogen, during the whole 35 hrs... we let the heater cool to 18ºC and got the following results at 500V

3.2 M Ohms

1.4 M Ohms

0.36 M ohms

0.4 M ohms

I then measured the heater at 200ºC

and got less than 0.2 M Ohms for all the four groups...

Weird... I was hoping to save us from shuting down our plant.. but I think this is more than just a wet heater problem.

Insulation breakdown?!! New elements. Hmmm. What is the insulating material in the element tube I wonder?

The terminal ends are clean and all ship shape? No cracks or gecko crap?

Maybe your elements were manufactured on a Friday after lunch...

Your supplier is at least communicating with you, be grateful for that. Keep asking him for help.

Do you have any other elements you can get your hands on for some comparative measurements? A forensic approach might give up a clue....or not.

New elements... magnesium oxide as insulator... im with you about the lunch bit... they were made in spain.. after a 3 hr siesta..what should I expect... lol:=

First of all, all insulation materials have NEGATIVE TEMPERATURE CO-EFFICIENT.

That means the resistance to insulation will drop with increase in temperature. I have seen in some cases ratio more than 10 between resistance at ambient and at working temperature.

Hence it is nothing strange that the value of IR at working temperature is much less than at ambient.

I do not think that you have measured IR of New Heaters at ambient and at working temperature. If done, it would have given you the clue whether insulation has deteriorated after some use.

Best is that:

Take present value as reference.

Set your protection to trip 10 to 20% more of present leakage current.

Understand your heaters are Delta connected, hence there is no chance of Neutral current mixing with earth leakage current and being sensed as earth leakage by your E/F relay. Even then better to check quality of insulation of Neutral conductor (if it exists). This is to ensure that return current of any single phase load is not mixing with leakage of heaters - in case neutral conductor insulation has ground anywhere.

In future if IR value further drops then the insulation is deteriorating otherwise it has stabilized at current level.

But humidity can not stay in the insulation once the temperature is above 100 Deg C.

Ok .. but now if the IR at 200 ºC is as low as 0,1 M ohm when the controller applies the full voltage to increase temperature to 280 or 300ºC we might have a problem.. My other problem is that these 4 groups are in the same heater .. basically we have 4 X 80 KW elements to make up our 320Kw Heater.. all 4 groups are in Delta and are all controlled at the same time.. why do I have diferent IR values to GND ..

Finally I have stolen this from "Google" "Magnesium oxide is an ionic compound and thus does not conduct electricity in its solid state because there are no delocalised electrons due to its ionic lattice structure. However when in its molten state, or dissolved in water tom form ions, it can conduct electricity as electrons can easily flow through.

Now if this is true then unless the insulation is melting it should not conduct.. hence the IR to earth should not really change.. or would it.. ?? like I said .. Im confussed..

Regarding your post that you found insulation properties of MgO on Google and want to know that why solid MgO is not a perfect insulator. You need to read further on Schottky effect and Schottky barrier and would know why with increase in temperature the insulation starts conducting more and more (Resistance starts dropping).

Coming to your problem, I read your post again and found something different.

Even at 200 Deg C, Minimum IR is 0.2M Ohm. This corresponds to 2mA of current per element at 400V to earth (actually it shall be less because phase to earth voltage is 239V for 3 phase 415V and neutral supply system). That means leakage of 3 legs shall not exceed 6mA. It is still within limit of 30mA of RCB relay.

If your RCB is tripping is most probably due to 3rd harmonics and its multiples being generated by chopping of voltage by controller. Therefore suggest:

1. Measure leakage current through Earth leakage relay using True RMS current digital multi-meter or am meter.

2. If this current is close to calculated from minimum resistance - and well within range of sensitivity of Earth Leakage relay, then connect a Low Pass Harmonic Filter in measuring circuit of Earth Leakage Relay.

Above may solve your problem because 3rd and its multiple are same as Zero sequence current for 3 phase supply, being detected by earth Leakage Relay.

As for the Insulation resistance of Magnesium oxide.. I agree that the IR drops with heat... I just couldn´t understand why.. You ve given me some Reading material ..Thanks....

Now what you say about the tripping also makes sense..´You have given me something to think about... Once again

Thanks

Apart from greater than 300mA and less than 1A what is the leakage current? Have you measured that yet porky?

PowersolutionsFBD suggestion has some merit to get things going.

How to explain why the elements don't meet the supplier's stated spec?

Spanish.....I'd best reserve my personal opinion.

The leakage current varries the highest I managed to measure is 720mA

So, PowersolutionsFBD recommendation would have you try setting it to 800mA.

How's that feel for you?

You could simply not ground the bugger.....and then have a 3 phase RCD set to say 30mA in case it tries to leak lethally through someone you like working with.

What's the voltage between the heater casing and ground with the ground cable removed? Can you light a lamp off it?

yip gona have to do that and pray that "this Humidity " disapears...

truth of the matter is that the IR valves have increased.. but what will happen after I remove the tape covering all the air vents and switch off the nitrogen in the junction box...

When first placed installed the M Ohm readings where just above 2 M Ohms for all the groups... and these values only increased after purging the JB with Nitrogen and running the heater for over 35 hrs...

Have you considered that the leakage current could comes from the thyristor controller? Are the snubbers grounded? MOV's damaged from testing it with a Megger?

Also, the fast voltage transients produced when the thyristors switch can induce ground current through capacitive coupling. If your ground leak detector is sensitive to high frequency, it may trip.

Good luck.

All IR measurements of the elements have been made with heating elements disconnected from the supply ( control panel)..