This week’s blog is about a testing experience I had some years ago while testing the resistance of Safety Earth Ground – This blog is taken from a section in my book on the subject and is in conjunction with the release of an updated version of my Book on Amazon – Grounding, Earthing, Bonding. The print version will be available also next week. The new, updated version has thirty pages of full color photos of do’s and most importantly the don’t. Shameless plug follows – (If you don’t own it, it might be a good time to buy it as the price will go up with the second edition.) And if you already own the Kindle copy, Amazon will notify you to download the updated version after it is posted.
As we know, most Earth Bond Tester will provide between 20 and 30 Amps at about 6 Volts AC to test the physical joint and also that the cabling used is adequate. Testing the resistance of EVERY Safety Earth Ground connection is essential and SHALL BE DONE during the first product safety testing program. (Production Testing is another matter and is outside the scope of this document.)
Here’s why we do it.
I was reviewing a cutting edge piece of biological analysis equipment in the Republic of Ireland.
These details are important because they indicate the high number of PhD’s, and cutting edge technology involved in the design of the product.
The prototypes had a fabricated chassis (about the size of a small car) – this was covered by a thin fiberglass shell with windows and doors.
In addition to the PhD’s there was a large number of very young inexperienced designers. Among other things they were designing mains circuits and making PCB’s (that had inadequate creepage distances) and other parts.
There were long lengths of square sectioned Aluminum Tube, Threaded Rods about 4 foot long, and buckets of nuts and washers. Some of the young (cheap?) labor were involved in ‘spinning nuts’ down the long Threaded Rods, it did not seem to be creative or taxing employment.
Before working on anything I always start by checking if the Safety Earth Ground is below 0.1 Ohm. (Self Preservation.)
I connected my Earth Bond tester – it showed an open circuit. I checked the connections and tried again – several more times. Then I did a “SANITY CHECK” using a multi-meter (DVM) set to “Ohms ÷100” – still Open Circuit. Then “Ohms” – same difference.
Finally “Ohms x 100” this would use a 15 Volt test voltage which should break down any ‘metal cutting compound’ or whatever was causing the problem. Not a sausage!
I might have believed that one of the instruments was defective but not both.
Then I remembered passing a strange road sign on my drive, there:
I was about ready to believe anything.
In desperation (and because it was the only piece of test gear I had left to try) I connected my HiPot (Insulation Tester) between the incoming Safety Earth Ground Green/Yellow Wire and the aluminum ‘bars’ of the chassis. That too was open circuit – I touched the HiPot leads together – There was an Impressive FLASH and everyone jumped back several feet.
Conclusion – the anodizing prevented ANY electrical contact and when I did a “Break Down Test” to discover the insulation properties of the client’s Grounding system – it took 1,450 Volts to break down the insulation. Any fault presenting less than 1,400 Volts would not be protected!
Over the years I have discovered a curious and inexplicable tendency for non-technical business owners to readily accept the craziest suggestions from highly educated specialists that know nothing about electrical design.
I guess because they rely totally upon the specialist expertise, that it seems safe to assume that if they have THAT much knowledge they must also be experts in ELECTRICAL Safety. There’s that ASSUME word again.
I never returned to that client. That village in Ireland is the only place on this planet I have used an Insulation Tester to check Ground Continuity – I survived that place once – I don’t want to push my luck by returning!