Myths Truths Correction Formulas20080211 WIND

7/31/2019 Myths Truths Correction Formulas20080211 WIND

1/3

Myths and Truths AboutWind and Load Correction Factors

2008 The Snell Group / www.thesnellgroup.com

Greg McIntosh, P.Eng.

The Snell Group-Canada

There are a number o ormulas and computer programs or

correcting or predicting what will happen to the

temperature (or temperature rise) o an electrical ault

when the load or wind changes. While these ormulas

are somewhat useul to indicate that signicant changes

in temperature may take place when the load or wind

changes, they are subject to much abuse and misuse

and may be downright misleading when it comes to

prioritizing repairs.

We like to ask thermographers who regularly use these

ormulas why they use them. Invariably their answers

include: to better predict the priority; to normalize one

ault against another; to correct a trend in a graph when

monitoring a ault condition; or to predict when ailure

may occur.

Indeed our Level I, II and Electrical Specialty classes are

witness to excellent experiments which illustrate theeects o convection and/or load in order to understand

why and how the ault temperatures should generally

change with increasing load and wind. But when it comes

to predicting, normalizing, or correcting a temperature,

or temperature dierence, there is a signicant danger

o severely under-estimating or over-estimating the

true ault temperature and the consequent severity and

priority.

Wind Correction

The misuse o wind correction mainly comes into play

when thermographers use inrared under conditions ohigh convection to permit them to continue and estimate

the temperature when the wind will be lower. There are

however many other variables, other than wind speed,

which aect the convective heat transer co-ecient in-

cluding wind direction (orientation), shape o the object,

surace roughness and whether the fow is laminar, tur-

bulent, or has separated rom the surace.

The change in the convective heat transer co-ecient

can increase signicantly when wind increases rom 0

to 15 mph and may, or a horizontal downward heated

surace increase more than ve times (1.08 to 6 BTU/hr-

t2-F), yet or an upward acing surace increases only 3.7

times (1.63 to 6 BTU/hr-t2-F)

The ASHRAE Handbook o Fundamentals shows that at

15mph the convective heat transer co-ecient changes

by 50% by moving rom a smooth surace to very rough

surace. (6 to 9 BTU/hr-t2-F)

Either o these two actors could easily represent more

than a 50% error in temperature calculation i an incorrect

assumption about orientation or roughness is made in the

correction actor.

While we could argue this complexity and even try to

correct or it using a more sophisticated model, the act

remains that we can be a signicant distance rom theenergized surace. How do we know the windspeed

we are measuring on the ground is the same as the

wind speed at the object, which can be many eet in

the air, and away rom any surace eect? Add to this

that the wind is constantly changing in velocity and

direction and we nd ourselves in a very uncertain place.

Load Correction

Fault heat generation varies with the square o the current.

There is no doubt that i load increases the component

temperature will rise. But by how much? Load increasealgorithms over-simpliy the real-world. The question you

should always ask is whether the surace temperature

being observed is the actual ault temperature or simply

an indicator o the ault temperature.

In all likelihood the surace temperature we measure will

not be the ault temperature. I the ault is beneath, or

behind the surace being detected with inrared, the 2nd
http://www.thesnellgroup.com/http://www.thesnellgroup.com/http://www.thesnellgroup.com/


2/3


Law o Thermodynamics states that the heat will fow

rom the point o highest temperature (the ault) to the

area o lowest temperature (likely the surrounding air).

Heat, like electricity, however takes the path o least

(thermal) resistance to achieve this fow. The surace

temperature we are observing may or may not be along

this path o least resistance.

In reality, whether it is a cabinet attached to a wall, a

conduit leading away rom a box, or the bus bar/

conductors leading out o the vicinity o the ault,

unknown parallel paths o heat fow will exist. The

pathway creating the resistance between the ault and the

surace we are observing results in a thermal gradient

which may or may not have a very large value i.e.: the

surace temperature may be quite dierent than the ault

temperature and any load correction ormula made to the

surace temperature may not produce a correction actor

even close to reality!

Thermal Run-Away

Another very important part o prioritization involves the

issue o thermal run-away. Many types o ailures occur

when a change to the system happens (load increas-

es, motor starts, wind decreases, ambient temperature

increases) and the thermal output o the ault increases.

At this point many variables come into play to dissipate the

additional heat generation including specic heat, mass,

surace area and paths o thermal conductivity. I this

additional heat generation at the ault cannot be dissipated

as ast as it is generated, the temperature o the ault increasesaccordingly.

The electrical resistance o many types o aults increases

with temperature, which in turn generates more heat and

a rapid ailure cycle starts. This is called thermal run-away.

There are so many variables unknown to the thermogra-

pher that predicting thermal run-away simply becomes

guesswork.

I predicting ailure is guessswork then prioritization

based on such assumptions can only be described as

irresponsible. One thing we do know is thermal run-away

most oten occurs at the worst possible time: at startup;

during switching; at peak loads; or at extreme ambient

conditions.

Alternative Strategies

One thing that is true about correction ormulas is that

they are dangerously convenient. Everyones job would

be ar easier i we could just plug in a number and nd a

solution. Unortunately, the only real answer to prioritiza-

tion is to say We have a identied a potential problem

and the more we do not know or understand about it,

the clearer we have to be that we simply do not knowhow bad it is or could get. So what is an alternative?

1) Ask yoursel whether you really need to quantiy the

problem at all. Based upon all actors such as

criticality, saety, spare parts, quality, reliability, etc., i

you do nd a problem (no matter what the temper-

ture or temperature rise is) the best course o action in

most cases is simply get it xed!

Example of Thermal Run-Away. Images taken over a 45

minute period during an inspection on May 22, 2003. The

strands were arcing in image 3 at 1:29 p.m.

Images courtesy of Doug Gerhold.
http://www.thesnellgroup.com/http://www.thesnellgroup.com/


3/3


2) Identiy whether there are other methodologies better

suited to diagnose this particular type o problem.

Motor circuit testing, impedance testing, megaohm

testing and oil testing are sometimes viable ault

validation tools.

3) Always take into consideration the possibility o

thermal run-away and the actors which may cause it.

I you are not sure run-away could happen, or whenit might happen, say so. An underestimated ault which

ails has cost the credibility o more than a ew

thermographers programs.

4) Go back and re-shoot the component under dierent,

more avorable conditions (lower wind, higher load).

5) Start trending the problem the less you understand

about the nature o the problem (and the severity) the

greater the initial inspection requency should be or

this particular component.

6) Gather other inormation that will allow you to

prioritize without using temperature as the only dete

mining actor. Consider all relevant actors

contributing to the thermal situation such as present

load, uture load, duty cycling, ambient temperature,

wind, and past history o the component to estimate

the probability o ailure. Combine this estimate with

the criticality o the component with respect to saety,

environmental consequences, eect on operations

and the cost o ailure and you have a very good

method or prioritizing each ault.

Finally, always remember that while correction ormulas

or wind and load may provide thermographers with con-venient thermal perormance indicators, unortunately,

they are not very reliable or prioritizing repairs. It is our

recommendation that you give strong consideration to

these alternative strategies when conducting inspections.
http://www.thesnellgroup.com/http://www.thesnellgroup.com/

Documents

Myths Truths Correction Formulas20080211 WIND