While teaching a group of electrical workers why one task is considered an arc flash risk and another task is not (even while performed on the same equipment under the same environmental conditions) we came to the realization that there is one key factor in arc flash risk assessment…
If you can determine the likelihood of an Arc Flash happening, then your risk assessment is 99% complete.
NFPA70E describes arc flash risk assessment as a type of risk assessment but they would be better off just leaving the arc flash bit out of the standard and sticking with plain old risk assessment.
They’ve defined risk assessment as “an overall process that identifies hazards, estimates the potential severity of injury or damage to health, estimates the likelihood of occurrence of injury or damage to health, and determines if protective measures are required.”
Let us take you through the steps of risk assessment (while keeping arc flash in mind) and it should be clear to you that the only step that really counts is the likelihood step.
Step 1: Identify the hazards
Ok… Arc Flash… check.
Step 2: Estimate severity
Bad, really bad, or extremely bad.
It doesn't really matter which it is.
In all cases, if you are exposed to this hazard it's not going to be pretty.
This is the reason why the likelihood becomes so important. It’s the only variable that is left to decide whether or not you are going to wear PPE or take any additional precautions to perform your task.
Step 3: Estimate likelihood
Ask yourself this question:
Is there a chance that what I’m about to do could cause an arc flash?
That’s it. Yes, or no.
Because the severity is so high, even a task that only has a remote chance of causing an arc flash needs to be considered risky enough to warrant the use of Arc and Flame Resistant PPE (or other precautionary measures).
Step 4: Determine protective measures
If you ended up with a ‘yes’ from step 3 then this is where you simply look at the Arc Flash Label to identify the amount of incident energy available or if you don’t have a label you’ll need to follow the “arc flash category method” that is outlined in NFPA70E.
Let’s recap the situation.
Your dealing with a possible arc flash, you never want to get burned no matter the available energy, and you'll always wear Arc PPE to protect yourself from the risk (that’s steps 1,2, and 4 for those of you following along).
The only time you do not wear PPE is when the likelihood of an arc flash happening is low, examples of these situations would be;
- In an office
- On de-energised lines
Still need help determining likelihood?
The NFPA70E technical committee actually makes it pretty easy.
They have a table called “Arc flash hazard identification for alternating current (ac) and direct current (dc) systems”.
In my opinion, they could have named it “the likelihood table”.
It’s just a big list of tasks that one might find themselves doing and a ‘yes’ or ‘no’ if they require Arc Flash PPE while they do it.
If you are stumped as to whether or not the task you are doing is likely to cause an arc flash, then see if you can find it or a similar one in the table.
How did the technical committee determine the likelihood?
They used the concept of normal operation.
To determine normal operation the asked themselves this question…
"Am I operating (or interacting) with this equipment in a manner that it is designed to be operated?"
If the answer is ‘no’ then you are likely to cause an arc flash.
For example, disconnects, switches and breakers are designed to be operated… so it’s unlikely those activities will cause an arc flash to happen.
Rack in rack out breakers are not designed (for the most part) to be racked in or out on a live bus… so the likelihood in this case will be high.