class of insulation and temprature rise

what is the differance between class of insulation and temprature rise in details ?

for example wat's the differance between






the first letter for class of insulation

the second for temprature rise

Super Contributor

Re: class of insulation and temprature rise

The first letter usually denotes the actual insulation system applied to the windings.


The second letter denotes the design temperature rise based on industry insulation class limits.


For example the H/B is a machine with a class H insulation applied to the winding but has a design temperature rise of 60 degrees C which is the specified maximum tmep rise for class B insulation.


Hope that helps,


Mike L.

Cat Dealer

Re: class of insulation and temprature rise

Actually, The insulation class decide the quality of Generator Winding and the temp rise is related to generator output rating.

And if the same winding insulation class but the less temp rise, then the life of the winding is longer, the life of generator is longer. But the price is higher.

 If we reduce the temp rise of the generator, it could be called Oversized Generator. 

H insulation class is higher and better then F insulation class.

Cat Dealer

Re: class of insulation and temprature rise



The first digit is the thermal limit of the insulation/generator which will allow it to run a predetermined number of hours statistically without failure of the insulation.


The second digit is what temp it is expected to operate at the suppliers rating (usually less than H).


H = 125 degC over 40 degC = 165 degC

F = 105 degC over 40 degC = 145 degC

B = 80 degC over 40 degC = 120 degC


If you have  H/H (125/125) the insulation will degrade quicker than an H/B (125/80). So how to apply this, well for instance H/H for standby and H/F or H/B for cont. The bigger the diffence is, eg H/B, the larger and greater the oversizing of the generator








Re: class of insulation and temprature rise

more details, pls

Re: class of insulation and temprature rise



Re: class of insulation and temprature rise

Expanding on Pierre's comments:


The temperature rating he describes is a continuous rating.  Manufacturers are allowed to operate alternators in standby applicaitons at up to 30C higher temperature rise.


Roughly speaking regardless of the rating of the insulation, the life of the machine will be approximately the same, as long as the insulation rating and operation/use does not cause operating temperature exceeding the temperature rating of the insulation.


As Pierre notes, when you apply an alternator at a lower temperature rise than its rating, you are effectively oversizing the alternator, which will result in longer life, better short circuit performance, better motor starting performance, and better voltage waveform when operating non-linear loads. In general, since the alternator is a small percentage of the cost of the generator set, it's a good practice to oversize a bit...for example using class H insulation with a class F temperature rise.  This makes the alternator perform more like the utility, so there is less potential for problems with operating loads.


Note also that the insulation class does not imply quality in any way:  You can have poor quality class H insulation systems that will not be as reliable or give as good life as class F.  So, you need to look to other factors to get to whether or not the insulation system is one with acceptable quality.

New member

Re: class of insulation and temprature rise


I need help. I have cat 3512 Generator 1030 KW,

The winding temperature up105 deg C and the generator trip/ shut down automaticaly.


can I set the limit winding temperature to the higher value, such as 115 deg C or what is the max I am allow to set?

please advise.




best regards,


tony lee

New member

Re: class of insulation and temprature rise

You need to look at the nameplate of the generator to determine the class of insulation that was used on the windings.  Then refer to the instruction book or other documentation to see if there were any guidelines provided as to the safe operating temperature.  Baring that, you can use the insulation class to back calculate the safety temperature by taking the temperature rating of the insulation (e.g. -- Class H is good for 220C) and deducting the maximum allowed ambient of 40C and the hottest spot allowance (it's 30C for transformers, I don't remember off-hand what the allowance for generators is); the resulting temperature is the maximum allowed operating temperature and a shutdown would logically be set just beyond this by a sensor tolerance, say 5C.


If you're already shutting down now, I'd also look at what you can do to help either unload the unit or improve its cooling:  ensure all area ventilation is running, any air filters are clean, no dampers are shut, there isn't excessive dirt in the unit, etc.  The other thing to check is that the relay that's getting the temperature reading is in calibration.


Remember, temperature is the enemy of insulation and attacks it by driving out the inherant moisture that needs to be there for it to work correctly.  The normal rule of thumb is that for every 10C rise in operating temperature, the resulting life is reduced by half. 


Re: class of insulation and temperature rise

This specification ( for instance class H with class B rise) became common from my memory starting in the mid to late 1960's, well before computer analysis of hot spots in winding design and VPI encapsulation ( a manufacturing method) were common.


Many independent consulting engineers and other specifiers felt it would extend the life of a generator insulation system for motors and generators because most of  the major electrical machine manufacturers in the USA had presented a variety of technical papers and published data that purported to show, based on laboratory testing  a 10 C increase in internal operating temperature of the insulating material above its base design temperature rating resulted in a 50% decrease in insulating material life. This premise was and is in my opinion significantly flawed, because it completely ignores the more significant insulation degrediation factors of atmospheric contaminants, moisture, dust, salts, oil, antifreeze, chemicals, etc that occur in real machine operation.


It is my opinion based on years of experience and client feedback that this requirement has never extended any electrical machine's life or overhaul frequency in the past 20 years or so, assuming the basic NEMA insulation class is "F".


Sorry for the detailed wording, but this key to the answer.