Why Mineral Insulated Heating Cables have power output limit?

[amrmohammed]

Hello

Why does Mineral Insulated heating cables have such limited power output per feet and maximum operating voltages?
I thought it has to do with the dielectric strength of the MgO, that is used as an insulation in the coaxial cable, but the dielectric strength for MgO is higher than the current voltage limit. The highest voltage limit I read about was around 2 KV and the dielectric strength of MgO is around 10-35 KV/mm. What happens if you supply power over the rated maximum?

 

[Bystander]

What's thermal conductivity of MgO?

 

[amrmohammed]

45–60 W/m·K (Wikipedia)

 

[Bystander]

Does the melting temperature of MgO establish an upper limit to operating temperature?

 

[amrmohammed]

No it doesn't, MgO has a very high melting point (~2,800 °C)

 

[Bystander]

No it doesn't

And, that limits the temperature gradient that can be maintained across the insulating annulus, does it not?

 

[amrmohammed]

that limits the temperature gradient that can be maintained across the insulating annulus

Would you please elaborate? how does high melting temperature of the MgO in the annulus be a limitation to the maximum temperature gradient?

 

[Bystander]

What's the purpose of a non-conducting solid insulator in the cables?

 

[amrmohammed]

To electrically insulate the heating element from the outer metallic sheath

 

[Bystander]

How well can it perform that function as a melt?

 

[amrmohammed]

it will fail, but it has a very high melting point

 

[Bystander]

it will fail, but it has a very high melting point

It sets an absolute upper limit to the temperature gradient that can be maintained across the insulation. (The practical limit is much less due to increased electrical conductivity of MgO as temperature increases.)
What does that limit to the temperature gradient imply as far as rate of heat transfer (power)?

 

[amrmohammed]

The limitation in temperature gradient should limit the rate of heat transfer, as additional power will increase the temperature of the heating wire and thus exceed the maximum temperature gradient to the surrounding. but does the change in the electric resistivity of MgO with increasing temperature that significant? most of the available heating cable have a maximum of only 600 V and MgO have a very high dielectric strength!

 

[amrmohammed]

Please refer to this pdf file. The resistivity of MgO do decrease with temperature but it is still very high.

and in http://www.emersonindustrial.com/en-US/documentcenter/EGSElectricalGroup/products_documents/heating_cables/commercial_heating_cables/commercial_pipe_tracing/mi_comm_pipe_trace_cable/308-SA-001_Mineral_Insulated_heater_cable_lit.pdf , the resistivity of the heating wire is presented and it is very small that it will always be much less than the resistivity of MgO insulation (using the graph above) so I though there should be no worry from short circuit, however they set a maximum temperature of 350 C and maximum voltage of 600 V. What is wrong with my understanding of this data and graphs?

 

[Bystander]

This Wiki article may help. https://en.wikipedia.org/wiki/Dielectric_strength Dielectric properties are measured under "ideal" conditions, and yield "best case" numbers that do not apply to "real" situations.

 

[Baluncore]

The lifetime of a resistor or fuse is reduced at higher temperatures.
For a resistive heating element, the cost of replacement following failure can be the factor that requires limits to the operating temperature.

 

[sophiecentaur]

I can't find any mention here,m of the effect of having a hot cable on its environment. (Setting fire to floor joists and melting nearby PVC cabling etc.) Surely that must be relevant. Or is it there 'by implication' in the rest of the spec of the cable?

 

[Bystander]

Heating elements ordinarily --- https://www.google.com/?gws_rd=ssl#q=mineral+insulated+heating+cable&tbm=shop

 

[sophiecentaur]

Yeah. I guess the design of the heater would take that into account. If the cable gets red hot then it would be mounted safely, by design.

 

[amrmohammed]

The lifetime of a resistor or fuse is reduced at higher temperatures.

That will limit the operating temperature (different from temperature gradient across the insulator that @Bystander mentioned), how is that related to the limit on maximum operating voltage? I guess the voltage limit should be somehow related to the dielectric strength of the insulator or the electrical properties of one of heating cable's components.

If the cable gets red hot then it would be mounted safely

That is true and there is a maximum sheath temperature set and published by IEEE. However, in many applications this is not a concern (like underground pipes and downhole oil heating) and the heating cable still have the same limit on the operating voltage and power output.

 

[amrmohammed]

Power is transmitted through coaxial conducting cables that is insulated by a dielectric material, could capacitance play any rule?

 

[Baluncore]

Power is transmitted through coaxial conducting cables that is insulated by a dielectric material, could capacitance play any rule?

Capacitance only becomes a problem above kHz frequencies. It is not important at 50Hz or 60Hz power distribution frequencies. Parallel capacitance is probably an advantage at power frequencies as it will slightly improve the power factor of the normally inductive grid.

That will limit the operating temperature (different from temperature gradient across the insulator that @Bystander mentioned), how is that related to the limit on maximum operating voltage? I guess the voltage limit should be somehow related to the dielectric strength of the insulator or the electrical properties of one of heating cable's components.

You seem confused.
The thread title is “Why Mineral Insulated Heating Cables have power output limit? ”
Maximum operating voltage determines electrical insulation requirements.
The durability requirement is determined by the maximum specified operating temperature of the inner conductor.

The mineral insulation needs to be both thermally conductive and electrically resistive.

 

[amrmohammed]

Maximum operating voltage determines electrical insulation requirements.

So to sum up, current commercial heating cables handle a maximum of 2 kV, because the currently available insulation (MgO) can not insulate higher voltages in field conditions?

 

[Bystander]

... and conductors (heating elements) with high enough resistance to function as heating elements (10-100 W/m) at applied voltages greater than 2 kV are too fragile.