End of life on electrical equipment. 1

[wolfie1a]

End of life on electrical equipment.
Looking for some feedback, opinions or any help on the end of life of electrical equipment (power distribution). Of course as equipment ages it becomes less reliable and I’m assuming less safe. With that in mind when do you start replacing equipment such as transformers, relays, Switchgear, MCC’s, distribution cables and other misc electrical equipment? I know duty cycle and maintenance plays a huge role in this discussion but I’m looking for any generic guidance. Any opinions, advice or other comments are appreciated.

 

[dbaird]

Don't know for sure on the power distibution products but due to the relative new EN ISO 13849-1 (PL, Performance Level)and EN 62061 (SIL, Safety Integrity Level)standards, some of todays controls have documented MTBF (Mean time between failures during normal operation), MTTFd (Mean time to dangerous failure)and MTTR (Mean time to repair i.e always significantly less than the MTTF values. I don't know if manufactures are absolutely required to provide this especially on their older devices. But I would think mission critical and / or safty related devices would and perhaps this will filter over into the power products as well.

David Baird
Sr Controls Designer
EET degree.

http://www.afctool.com

 

[KllrWolf]

I would think items, such as switches and relays, that have moving parts would have an established life based on cycles of operation. Something such as a transformer should not need replacing (rewinds probably) unless something catastrophic happens to it or there comes along a new design with an efficiency great enough to warrent replacement. Of course I am projecting my experience with electric motors onto the transformers.

 

[cranky108]

Relays should be rated in the number of testing cycles they have endured. It would be rare to have a great number of faults on a relay, unless your system has loads of other problems.

Look at the bath tub curves for the equipment you are wanting to replace to see what the life of simular equipment is. Then like transformers that may not be the actual end of life based on the possibility of rewinds. Most equipment should last over 30 years, except the electronic equipment, which should be more like 15 to 20 years.

We removed a set of relays after 75 years of service, not because of any failures, but because of the Hg in the contacts.

 

[ScottyUK]

KllrWolf,

Transformers most certainly do age. The principal problem is that the cellulose-based materials such as pressboard and Kraft paper gradually de-polymerise and as the cellulose chains shorten the mechanical strength is slowly lost. It is entirely possible for a winding to burst under heavy through-fault conditions. Paper within condenser bushings deteriorates too, but generally at a much lower rate than the main insulation. Oils also degrade: as cellulose breaks down it evolves water which enters the oil. It can be removed to some extent by processing with heat and high vacuum, but much will remain in the core unless a very long drying cycle is undertaken. Some fault conditions can cause the oil itself to break down, forming acidic compounds which further reduce the life of the paper, and releasing gases.

All of the above is affected by temperature, load profile, maintenance programme, etc so there is no 'correct' answer for expected service life. Some transformers with severe service conditions and no maintenance are in trouble at 20 years old, others with easier duty and good maintenance are nearing their centenary year.

 

[DTR2011]

Here is a good place to start. Link

 

[rhatcher]

Proper testing and maintenance is the key to determining end of life for power tranmission products. For motors and transformers, at a minimum, insulation resistance tests should be performed and trended. When the trend turns down the life is known to be near the end.

For MCC's, insulation resistance of the bus bars and IR testing of the starters is a minimum. When the insulation resistance of the bus bars decreases to a minimum level or when the amount of starter repairs indicated by the IR testing becomes impractical then the practical life is at an end.

For switchgear, insulation resistance of the busbars and testing of the breakers (insulation resistance, contact resistance, and overcurrent testing) is the indicator.

In a general case, proper maintenace and testing is the indicator for when something should be replaced. As long as the testing indicates that the device is reliable and the maintanance to keep it reliable is cost effective then you are good. When testing indicates a failure or the cost of maintenance and repairs becomes prohibitive then replacement is necessary.

 

[KllrWolf]

ScottyUK,

Thanks for that information, it is good to learn something, a star for you. The majority of the motors I deal with are full of water, so water intrusion is not an issue. When we design oil-filled motors, we work to use all the same parts we can from the water filled (winding wire and maybe bearings the only real change), and due to the conditions in which they operate, probably get more frequent oil changes than my cars, so the issues you mentioned are not something I would have thought about.