The Two “8 ms Rules” in Blasting: Why They’re Different and What It Means Today
At Petr Explosives Group, students in our education classes often ask: “Why do people talk about two different ‘8 ms rules’ in blasting?”
It’s a smart question! Both rules use “8 ms,” but they are not the same. They solve two completely different problems. Mixing them up can cause poor rock breakage, excessive vibration, or regulatory issues.
In this post, we’ll explain both rules in simple terms and show how modern electronic detonators are changing the way we blast.
1. The Vibration 8 ms Rule – “How the Seismograph Sees the Blast.”
This rule is based on vibration monitoring and government regulations. Its job is to protect nearby houses, buildings, and structures from ground shaking.
Simple idea: If two or more holes explode within about 8 milliseconds of each other, the seismograph (the machine that measures shaking) treats them as one big explosion. This makes the vibration stronger.
To keep vibration low, blasters must make sure not too much explosive goes off in any 8-ms window. This is used in the Scaled Distance formula to calculate safe vibration levels.
In short, the vibration 8 ms rule says “Don’t let the energy lump together” — because lumping energy creates more shaking at distant locations.
NOTE: Keep vibration under control and stay compliant with the law.
2. The 8 ms per Foot Rule – “How the Rock Sees the Blast.”
This is an old field rule used for designing the actual blast pattern. It says: Delay between rows = 8 milliseconds per foot of burden.
Example: If the burden (distance between rows) is 10 feet, wait 80 milliseconds before firing the next row. This rule was created long ago when detonators were not very accurate. The long delay gave the rock from the first row time to start moving (called burden relief), so the next row could break it properly — not too fast (which causes choking) and not too slow (which gives poor breakage).
In short, the 8 ms/ft rule says “Spread the energy out,” so each row has space to do its job and move the rock nicely.
NOTE: Get good rock fragmentation and a nice muckpile.
Easy Way to Remember the Difference
We teach this simple explanation in every PEG class:
- The 8 ms vibration rule is how the seismograph sees the blast → It lumps energy together. Protects buildings (compliance)
- The 8 ms/ft design rule is how the rock sees the blast → It spreads energy out. Breaks rock effectively (production).
Why the Old 8 ms/ft Rule Doesn’t Work Well Anymore
Here’s where it gets interesting. Stress waves (the fast energy ripples from an explosion) move through rock very quickly — 3,000 to 5,000 meters per second.
For a 10-foot burden, these waves cross the gap in only 0.6 to 1.0 milliseconds — faster than the blink of an eye!
But the old rule waited 80 milliseconds — about 80 times longer than the wave needs to cross.
So what happens during those 80 ms with electronic detonators?
- In the first 1 ms: Waves shoot across and start cracking the rock.
- In the next few ms: Waves bounce back and forth like echoes, loosening the rock.
- For the remaining 70+ ms, the rock from the first row is already moving and being pushed by the explosion gases.
By the time the second row fires at 80 ms, it is shooting energy into rock that is already moving and broken. It’s like trying to push a door that is already swinging open — you waste a lot of energy and don’t get the best breakage.
Modern electronic detonators are extremely precise (±0.01 to ±0.1 ms). Every hole fires exactly when you want it to.
This means:
- You no longer need the big “safety cushion” of 80 ms.
- You can choose the perfect timing so waves from different rows work together.
- You can make waves add up for better breakage or cancel out to reduce vibration.
NOTE: Instead of following a fixed old rule, you can now design blasts using real physics and a simple timing ratio based on your rock type, bench height, and goals.
The PEG Perspective
At Petr Explosives Group, we teach that the old rules were useful in their time, but they were simplifications. The 8 ms vibration rule is still important for safety and compliance. The 8 ms/ft design rule was a helpful compromise when timing was inaccurate.
But with today’s electronic detonators, we can do much better. We help blasters move from “follow the old rule” to “understand the physics and design smarter blasts.”
Our education classes make this transition clear and practical — so you get better fragmentation, lower vibration, and more consistent results.
Ready to Blast Smarter?
If you’re still using old rules of thumb, you’re not getting the full power of modern electronic detonators. Join our upcoming Petr Explosives Group training programs. You’ll learn to clearly separate the two 8-ms rules, understand wave timing, and design better blasts with confidence.
The future of blasting is not about fixed numbers — it’s about controlling energy at exactly the right moment.
Contact Petr Explosives Group today to learn more about our classes and hands-on training. Let’s build better blasts together!
