In an industrial setting, personal protective grounding is something that can be easily overlooked. It is also something that is widely misunderstood.
In fact, in most places temporary protective ground cables are not even thought of as PPE, but that is exactly what they are! Notice the title refers to personal protective grounding, well you are the person and it’s for that very reason you need to understand these next few concepts.
Why is Personal Protective Grounded needed?
If you are attempting to establish an electrically safe work condition you need to apply personal protective grounds to each phase of the system, as close to where the work is being performed but what for?
Well, with systems rated 750 volts and above there are actually a number of reasons you are going to want to do this, all of which can lead to a deadly shock or arc flash.
1. Accidentally Re-energized Circuits
The first and most obvious reason being that the circuit could be accidentally
re-energized. Maybe there are remote operators that close the circuit after you have already completed your voltage tests. Or possibly the bus you are working on is connected to a diesel generator.
2. Chance of a Lethal Shock
Secondly, induced voltages and currents are a very real possibility in an industrial facility. With plenty of cables running alongside one another as well as other high-voltage equipment in close proximity, there is potential to receive a lethal shock even after the equipment is disconnected from the system.
3. Equipment Insulation can fail
Over time the equipment starts to break down and current will start to flow where it is not supposed to.
How does personal protective grounding work?
The basic concept is that once you have de-energized a circuit, opened the disconnect, visually verified, applied locks and tags and tested for absence of voltage you still need to apply personal protective grounds before you can really say (for reasons listed above) the circuit is de-energized.
The sequence of applying the grounds is very important!
Always start with the grounded end first, then connect the phase conductors starting with the phase closest to your body and working away. When you go to remove the ground cables make sure you do so in the reverse order of what I just described.
So now that I’ve briefly described how to get the grounds on…
- How do they work?
- What if someone accidentally turned the power on?
- What is going to happen?
Well… this might sound crazy, but you just installed those ground cables in order to ensure that if the system was re-energized you would get the absolute most short circuit current to flow through the system as possible!
What? The most current possible?
Yes. There are a couple reasons for this.
If you have properly installed the personal protective grounds and they are sized correctly for the system, then they are going to create “the path of least resistance” for the fault current. This is good. The first thing is that you will not be the path of least resistance!
Secondly, it maintains the voltage at a safe level until the upstream protective devices can trip the system. Ground cables are very durable but there is a limited amount of time that they can withstand extreme amounts of current. The faster the current is interrupted the more likely the ground cables themselves do not become the hazard… remember you are probably standing very near when this happens.
Any other considerations?
As with any other personal protective equipment, personal protective grounds require periodic inspections and do need a certification to prove that they can do the job. If a set of ground cables was exposed to a short circuit - they do need to be tested and re-certified, before you can rely on them again.
And do not forget that the ground cables need to be sized appropriately to the system that you are working with. For example, a maximum fault current level will be much different between a 13.8kV and a 600V piece of equipment and those two systems would call for different specifications for your personal protective grounds.