When asked what I do for a living, I usually reply, “I draw circles, lots of circles.” But what are those circles? What do they mean? Why do we care? And why are there so many of them?
Explosive Safety, like all “safety,” isn’t always safe. It is a predetermined, acceptable level of risk. Like other areas of safety, that acceptable level can vary depending on an individual’s required involvement with the activity or object at hand. In the US, the Department of Defense uses four primary types of relationships to determine separation criteria. Of course, there are exceptions to everything, but today we’ll discuss those primary four relationships, what they are, and in general who is permitted to be there; so you can understand the circle-drawer at your next charrette.
First though, we need to discuss what the relationships are between. Any location, facility, or activity that has energetic materials is called a “potential explosion site” or PES. Anything an explosion could impact is called an “exposed site” or ES. An ES could be a road, it could be an office building, it could be utility lines; it is quite literally anything that could be hazarded by the PES. And to make things more confusing, an ES can contain explosives causing it to be the PES in a different relationship. These variables drive us to assess the relationships from every individual PES to every individual ES it could impact, which is why there are so, so many circles.
Naturally, the closer you get to explosives, the more hazardous the situation. So, we’re going to start off at a “safe” distance and work our way in.
Inhabited Building Distance (IBD)
Our first relationship is Inhabited Building Distance (IBD). Inhabited building distance is what you’ve probably seen displayed on planning maps as the Explosive Safety Quantity-Distance (ESQD) arcs. IBD is, in general, the greatest amount of protection the DoD requires, which is why it’s the arc most often put on the map. If you’re outside IBD, you’re good (usually, there are always exceptions)! This relationship is to personnel or facilities unrelated to explosives and which are more permanent in nature. Things like office buildings for administrative personnel, the dining facility, the local school, or really anything we want to provide a high level of protection to, would qualify for IBD. This is the default level of accepted risk (by the DoD) from explosives to anyone.
Public Traffic Route Distance (PTRD)
Moving a little closer to the explosives, we have Public Traffic Route Distance (PTRD). PTRD is the exposure to personnel who are not related to the explosives but are transient. Due to the temporary nature of their exposure and the inherently reduced risk of not being present most of the time, this traffic is permitted to be closer than personnel who would be permanently exposed. As the name implies, most traffic routes are permitted to be at PTRD. However, if the traffic along a particular route, such as an interstate, is too heavily populated, the relationship may be elevated to IBD. In addition, open air recreational areas such as running trails or golf courses are allowed to be placed at PTRD, as long as there are no permanent structures. Things like bleachers, clubhouses, or even the small bathrooms on the back-nine are considered permanent, so they require IBD.
IBD and PTRD are “exterior distances” because they are all non-related to the explosives. You can think of them as outside the munition storage area. Our next two relationships move inside the bomb-dump, and are “interior distances.”
Intraline Distance (ILD)
Intraline Distance (ILD) accounts for our need to have humans working with explosives. There’s little we can do to separate humans from the explosive they’re actively working on, but we can provide separation from other storehouses or operations nearby. ILD is this relationship between a PES and other personnel who work with explosives. It provides enough separation that an accidental detonation is unlikely to propagate, so if the storehouse explodes, it’s unlikely the assets in the operation will also explode. It is also the distance at which blast overpressure is roughly 3.5 psi, which is relatively low risk to the human body. At this distance, fragmentation is the highest hazard to personnel and barricades are desired whenever possible.
Side Note: Barricades do very little to protect personnel from blast over-pressure (unless the person is VERY close to the barricade). They are primarily (almost solely) for mitigating fragmentation hazards.
Intermagazine Distance (IMD)
Lastly, our journey towards danger brings us to Intermagazine Distance (IMD), which is our smallest separation requirement. IMD is the relationship between one pile of explosives and another. At this distance, were an explosion to occur, it could propagate to the second pile of explosives, but there would be a large enough time delay that the blast over-pressure would not be additive. In nerd speak, they do not coalesce, but in layman’s terms, it provides enough separation to have two small explosions instead of one large one. This can make a BIG difference in required IBD. By providing adequate IMD, we can spread our munitions over multiple storage locations and significantly reduce the overall hazard.
So, why bother with all these relationships? If IBD is “safe,” why not just use IBD for everything? Federal planners’ favorite topic, of course: to help maximize land use! On the upper end of extreme, the DoD allows a maximum of 500,000 lbs. of explosives in a single storage location. The required IBD separation for 500,000 lbs. is 3,969 ft; that’s three quarters of a mile. The arc would cover over 1,100 acres! At the other end of the spectrum, the general minimum IBD arc is 1,250 ft. This is significantly smaller, but that would still require over 100 acres per storage facility! Allowing other storage sites and functions to be closer to the storehouse allows us to utilize land more effectively.
While we only examined the relationships used by the DoD in this article, some foreign nations have additional relationship criteria. For instance, NATO uses an additional exterior relationship called Vulnerable Building Distance that’s even larger than IBD for particularly sensitive locations or structures. If you’d like to learn more about the caveats of explosive safety, contact The Schreifer Group about training opportunities or check out this article on What Should a Federal Planner Know About Explosive Safety.