Did you know that one of the most common reasons to get a question wrong even when you’ve done all the right steps is by making a mistake on the units?
I bet that sounds familiar.
Have you ever worked a problem that had all sorts of different units and you either entered it into your calculator wrong or misinterpreted the units of the answer that the calculator gave you?
What about when you are solving a problem and all the possible answer choices are the same value but in all different units:
We are going to make sure we never get an answer wrong because of using the wrong units, ever again!
After all, every single point on the Electrical PE Exam counts towards a passing score.
So let’s say you’re working a fault current problem and utilizing the KVA method.
You have an upstream generator with a fault power contribution of 3,000bMVA feeding a transformer with a fault power contribution of 820.9 MVA. You want to find the fault power downstream of the transformer so you add them in parallel like this:
Using powers of ten for the MVA units above, or using a whole bunch of zeros on our calculator like this:
Both give the answer in the correct number of zeros or correct power of ten for the units the answer is in when you hit enter:
Depending on how you calculator is step up, it will show the answer in any of the three ways above.
Now, for the good stuff.
I want you to know there is a better way.
These MVA values are already quite large so entering them in the calculator using scientific notation or with a bunch of zeros is time-consuming.
It also introduces additional steps where a mistake could be made.
Instead, I want you to:
- Look at how the units will cancel each other out
- Know what kind of units the answer will be in BEFORE you solve the problem
- Enter in the less amount of numbers in the calculator as possible
These steps all contribute to reducing the chance of making any costly entry mistakes that gets us all from time to time.
So how do we do this?
The method works like this.
First, let’s work the problem using ONLY the units.
Here is the same reciprocal of sums method again for adding our two Apparent Power fault contributions in parallel, using only the units:
This tells us the answer will be in MVA’s. So we write down the units for the answer before we even know what the value is.
This way we don’t forget it and it prevents us from making a mistake, like this:
Next, we enter in the calculator the numbers ONLY without using powers of ten or extra zeros for the units and hit enter. getting the answer without the correct number of decimal places, like this:
This gives us the answer without the correct number of decimal places for the units.
Here is the same example as above, this time using the MVA values of the fault contributions:
Then we write down the answer from out calculator into the spot we already have on our paper waiting for it, like this:
This way may seem longer at first, but I promise not only will you save precious time for each equation using this method, but it is also a great way to check your work when you need to double check if the units of your answer are correct.
It saves time on each question that can be applied toward the end of the exam to spend on harder to answer questions that require using your reference books.
Another point to consider is that after you do this a couple of times you will instinctively know the correct answer of your units, skipping the step of physically writing them down to see how they cancel out, and jumping straight to entering in the least amount of numbers for each value in your calculator.
It really does save you time and prevent mistakes.
How else can we use this trick?
let’s take a look at other electrical formulas where we have a mix of units or large decimal places and are interested in entering the least amount of numbers into the calculator to save time and cut down on additional steps that may lead to simple mistakes.
For example, let’s say we are calculating the magnitude of impedance from voltage and power:
If your voltage is in kV and your power is in kVA, what units will your impedance be?
Getting the hang of it now?
What about the units of line current in a three system with an apparent power in MVA and line voltage in kV?
The possibilities are endless, and you can apply this to just about every single example problem while you study and more importantly on the actual PE exam.
Thoughts? Further Questions? Examples of when you have used this successfully?