ECN Forum Forums General Topics of the Trade General Discussion Area Measuring resistance between bonding...

Say that you have this pool that is 5 years old and is dry for maintenance. Thhere is a ladder to your left and a diving board sopport to your right and you want to make sure that they are still bonded effectively to each other. You just bought one of those fancy-shmancy low resistance ohm meters that'll read micro-ohms, and what now...
Is there any maximum reading before you need to redo the bonding? Is there a standard? Who publishes a standard for bonded pool equipment? The closest thing I have found is <=0.00033 ohms across joints in a metallic cable tray system published in NEMA standard VE 1-1991 "Metallic Cable Tray Systems". We want our pool to be safe and bonded, so this is why we're looking at more than a visual or Fluke 87 based reading...

You may be well aware of this, but for the sake of others a critical part of the measurement is a Kelvin connection, (2 current and 2 voltage spikes) to automatically compensate for lead resistance.

Use a 10-ampere test current and compare readings of several mechanically similar joints in the pool. Maybe compare readings of one pool to those of other [newer?] pools, and establish baseline readings. Without previous readings, there’s not a lot else to do.

BTW, millivolt readings with a constant-current source and an 87 are perfectly good too. I've used 1 to 100 amperes for this job. They are still Kelvin measurements.

We're using a AVO DLRO10X that is a constant current source measurement. What if the bonds in a new pool are worse than the bonds in my 5 year old pool? What is a good bond? What is a bad bond? 25 ohms? 25 milliohms?

I don't think it's gonna' be chiseled in stone anywhere. For lack of anything better, what about comparing readings to that of ten or a hundred feet of 6AWG? There may be some numbers in IEEE 837, but that's for substation ground connectors. It may be reasonable to take readings now, and check them again in a year.

IIRC, the NEC used to suggest "acceptable" megger readings, but later they were tossed out because some interpreted them as mandatory, and that wasn't the intent of the code panel. Within obvious limitations, changes in readings may be more revealing of potential degradation than a one-time reading without any baseline data.

Many agree that the nominal three-point 25-ohm ground resistance is OK in a many cases; in other situations that reading would be criminal.

If you are used to seeing ductor readings, you are probably are more qualified to evaluate numbers than many others.