Thermographic (IR) surveys

[davva]

Is there a reference document that defines Classification of Severity for electrical equipment that is subject to thermographic surveys

i.e.
0-10degC above ambient corrective measures required at next maintenance

10-30degC above ambient requires urgent action to be taken

40+ urgent

If there is such a document does address the acceptable rises that are allowed for all types of electrical equipment, e.g. cables, busbars, connections, etc.

 

[davva]

Since posting I have found MIL STD 2194 which seems a good reference, but would welcome any other links/advice

 

[eyec]

assuming that you have baseline readings to compare the results to.

differentiation/comparison from like circuits/equipment can be just as informative given identical load conditions.

all of this should be confirmed with an experienced visual examination.

just my .0002 cents worth

Steven C
Senior Member
ThirdPartyInspections.com

 

[Zogzog]

Table 100.18 of ANSI/NETA standards.

 

[prc]

For power transformers,there is IEEE standard C57.140-2006
"Guide for the Evaluation and Reconditioning of Liquid Immersed Power transformers." Table 3 Suggested temprerature rise recommendations

 

[ausphil]

We always work on a temperature rise above another phase carrying the same current (comparitive thermography), rather than taking readings of the equipment above the ambient. This tends to eliminate any issues of temperature rise above ambient which can lead to false problem identification, especially when you may have a high load current, which can raise all phases joint temperatures well above ambient. You are comparing like with like (ie same current through the same joint) which allows you to easily identify problems. This works well at distribution and transmission level equipment, where you have roughly balanced loads, but where you get into customer's installations, you have to be aware that each phase may not be carrying the same load, so you may have to do some calculations based on actual load current in each phase.

We have tailored and used the limits below with very good success:
5-<10 degree rise - re-examine in 12 months time.
10-<20 degree rise - re-examine in 2 months time, or shorten scanning frequency on ongoing basis.
20-<30 degree rise - re-examine in 2 weeks.
>30 degree rise - immediate action.

Again, these temperature rise values are for comparative work, that assumes one or two healthy phases to use as the reference temperature. They are not a temperature rise above an ambient.

ausphil

 

[Zogzog]

Always better for follow nationally recognized specs, this NETA table is now a recognized ANSI standard.

When making recommendations to anyone you need to have a nationally recognized standard to reference to CYA in case something ever happens. Otherwise you have nothing to stand on.

 

[davva]

We were given (what I thought was) a pretty poor report and it just highlighted the fact that the conductors (all 3 phases were at 42degC). Their report statement was "possible conductor undersizing or overload". Ambient was 26degC, i.e. 16degC rise above ambient.

The problem was there was no record of what current was passing at the time and no mention of the cable size.

To me it seemed like it could have been normal operation? PVC cable insulation can run upto 75degC and EPR cable insulation can run up to 90degC, so I'm not sure what temperature you would get when you point a thermal camera at the outer surface of these cables when they are fully loaded?

This is why I started to look for standards or documents that say whether "something is hot or not". It didn't seem right that you compare something against the ambient.

As an example you may size an uninsulated busbar system to run at 90degC (with a basis of 45degC rise above a 45degC ambient). So if you pointed a camera at them in a 25degC ambient it is possible that you may see 45degC temperature rise above the ambient (70degC actual) when they are fully loaded.

It would seem that the "relative to" (relative to another conductor, conducting the same current in the same conditions) would be the better approach?

 

[eyec]

something often overlooked by novice thermographers is the emissivity factor. this can often lead to false readings.

that is why a visual is as important when doing an IR scan/survey.

signs of heat overload can be seen as an indication of a problem no matter what the IR scan shows.

Steven C
Senior Member
ThirdPartyInspections.com

 

[Zogzog]

We were given (what I thought was) a pretty poor report and it just highlighted the fact that the conductors (all 3 phases were at 42degC). Their report statement was "possible conductor undersizing or overload". Ambient was 26degC, i.e. 16degC rise above ambient.

Bet they were low bidder

THere are a lot of yahoos out there witha cheap IR camera that have no clue what they are doing, always be sure to get a certified thermographer.