Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations The Obturator on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

concrete as an insulator for the floor of an electronics shop? 3

Status
Not open for further replies.
drewbr549

drewbr549

Electrical
Aug 16, 2006
12
Hi all. I'm setting up an electronics bench, and there is a room in the building I'm renting that has a concrete floor (actually, it also has concrete walls and (what at least seems like) a concrete ceiling, as well). So, here are my two questions:
1) I don't own a megger. Can I use the leakage pen on my variable AC power supply (it's a bk 1655) to test leakage to ground to determine if the resistance/resistivity of this particular slap of concrete during this time of the year will provide sufficient insulation ( bearing in mind all of the considerations of thread248-29644 )? If so, what kind of numbers am I looking for?
2) Assuming, once again, the values given in thread248-29644, would concrete, in general, make a suitable flooring for an electronics bench floor, or should I invest in additional insulation? Unfortunately, I do not know the thickness of the concrete - would/could anyone recommend a minimum thickness (I'm in Missouri, and it's really stinking humid, if that's any help:))
Many thanx,
Drew
 
Replies continue below

Recommended for you

Sort by date Sort by votes
If you are talking about, "if I hold a hot wire while standing barefooted or in leather shoes or touch a metal bench standing on the floor will I die or be shocked?" Yes! Plan on it!

Concrete should be considered dangerous in that regard.

I bumped a 120Vac hot with my elbow while standing on dry 40 year old painted concrete. It knocked me on my ass.

Keith Cress
Flamin Systems, Inc.-
 
Upvote 0 Downvote
Concrete is not insulating. You can not use a simple test probe to test the insulation. It is done with a defined surface probe, a defined pressure and after a few hours (sometimes day) of "getting used to" (humidity equalization) period.

Paint will not help much. There are two ways to handle the problem: 1) Use a thick PVC or similar mat. 2) Use an RCD (Earth Leakage Breaker). The former is safe until it gets damaged. It may cause ESD problems. The latter is easy and you will not have any ESD problems (not likely in Missouri, anyhow). It is also safe. But there is the slight problem that it may disturb other guy's work while it is saving your life. I, personally, would be more concerned about saving my, or other's, life.

Go for RCD. But do not forget to test it!

Gunnar Englund
 
Upvote 0 Downvote
I was wandering through an old electrical linemans book years ago and found an interesting item. It said for a superior ground to encase the copper ground rod in 8 inches of concrete. The moist concrete effectively increases the surface area of the rod. I saw this effect when I was working on an engine test stand with a massive concrete base. It was the best ground around and the ground currents from the "utility ground" to the stand caused a lot of problems.

Working on concrete all day will get to your feet. Saw an interesting product at one of the big box hardware stores. Two foot square floor panels that linked together. They had a plastic base riser and wood top that would make a warm insulated floor. Rubber mats with holes are good, but a pain to keep clean.
 
Upvote 0 Downvote
Two kinds of moisture in concrete. Most is chemically bound, as hydrated oxides. There is also physically absorbed water, which will diffuse via capillarity from the more humid to the drier side. But, getting to the point, do this test, ASTM D4263, for (absorbed) moisture in concrete: Duct tape an 18x18" sheet of polyethylene sheeting to the floor and leave 16+ hours. Check for condensed moisture on the sheet.
Moisture's (almost) always present on slab floors, but not always on upper stories.
 
Upvote 0 Downvote
Why do you want insulation? Sufficient for what? Usually conductive, grounded surfaces are desirable for electronics work for the reason Skogsgurra noted. I suspect your variac secondary is isolated from ground anyway. Plug it into a GFCI receptacle to be extra safe.

In Missouri, RCDs are known as GFCIs. They are available at any hardware store, and can replace an existing receptacle.

 
Upvote 0 Downvote
Stevenal,

An isolating variac? I haven't seen one of them - all of our European ones are autotransformers. Are the US ones inherently isolating, or do they need a separate isolation transformer?

In the development lab I worked in we used metal-topped workbenches bonded to the power earth. Most were covered in 1/8" aluminium. One was topped in [¼]" brass with a hardwood frame, and must have been a lot older than I. It would have cost a fortune to build today. If we were doing any live testing on mains-powered circuits we would cover the bench top with an insulating mat. The mat was really intended for use on a switchroom floor so it was as tough as old boots and about 10mm thick.

We very rarely lost anything to static on those metal benches, even small-signal MOSFETs.

The RCD / GFCI is definitely a good idea. In Europe there are several types. 30mA instantaneous trip is probably the most common, but they are available with 10mA / instantaneous trip too. The latter type would be a good choice for a test bench.

----------------------------------
image.php
Sometimes I only open my mouth to swap feet...
 
Upvote 0 Downvote
Wow! Thanx, everyone, for your comments - every post was informative. I don't usually say this, but I learned a lot about concrete today.
With regard to your comments:

1) Yes, itsmoked, that is sort of what I guess I was wondering. I know I need to protect the path across my heart (is it one hand behind the back, HV gloves, or using both hands w/ one in a glove - I guess there's probably already numerous posts on this topic), but I was hoping that if I did that somehow that the concrete floor would keep me from turning the path from my hand to my foot into a human fuse. I've had the tingles a few times, but have never been knocked on my but - thanx for the warning.
2) skogsgurra, I am forced to confess that I didn't totally get the first part - I guess I always thought you just took your meg-ohm meter and poked around when setting up a shop. Could you elaborate?
The second part is a great idea - I definitly will. But, while we're on the subject - is there a difference between an earth leakage breaker and an ordinary pwerstrip breaker? Do they both just trip in the presence of excess current, or does the leakage breaker work differently. Also, is there a difference in how fast they work? Sorry, lots of Qs, but I probably ought to get this part right:) Also, thanx for the ESD advice - I hadn't considered that as a con to some kinds of matting.
3)OperaHouse and kenvlach, thanx for the water stuff - I guess I just always thought of concrete as being fairly dry and solid. Thanx also for the riser idea and the moisture test.
4) stevenal - many thanx for the term conversion ("...ah, a gfci - now I get it"). As far as why I want insulation - well, that's just how I've always read that you do it. It is the advice given in the book I'm reading right now, Practical Electronics Troubleshooting by James Perozzo. Anyway, wouldn't you want to keep your body from completing a circuit from Vcc to earth ground? I guess I don't get it. And yes, you are right, the 1655 does have an isolation transformer. I know it will protect me from some hazzards (primarily a hot chassis), but I didn't think I could count on it for any kind of generalized protection. This might be bc I don't totally understand the theory of operation, I mean, I (certainly) understand how a transformer works, with its primary and secondary(s), and how it changes voltage. What I guess I don't totally understand is how it keeps the chassis (i.e. the circuit common) from being hot w/ respect to earth ground. I mean, I know it's isolated, but it still passes voltage and current, so what's the difference whether it's there or not?
Anyway, thanx again to all for the great responses,
Drew
 
Upvote 0 Downvote
hey - I guess I took a long time to type that - there's already another up! ScottyUK, I've heard that there is a diff (i.e. that you can't just buy an old variac and expect to be protected) but I couldn't tell you why - I'll leave that part to the others. But, thanx for the clear up on why you would use a grounded surface in a shop. But, I was wondering - is it primarily to eliminate static discharge (like the cheap thingies you're supposed to wrap around your wrist), or does it provide additional safety to the technician?
 
Upvote 0 Downvote
never mind, that was dumb - you said you used an insulating mat when you had live voltage - so I'll assume the conductive, grounded bench was primarily to discharge static.
 
Upvote 0 Downvote
A circuit breaker is about protecting wiring and circuitry from catching fire.

Your powerstrip breaker is probably something like 10, 15, 20 amps, while a GFCI trips at 50mA?, and is intended to protect YOU from catching fire. ;-)

Additionally, concrete is a bit of pain to sweep, particularly if you get little bits of leads and wires on the floor.

Some sort of ESD matting would make that task a bit easier, protect your circuits from ESD and protect you from catching fire.

TTFN



 
Upvote 0 Downvote
wow, several answers, but still very clear and concise - thanx!
 
Upvote 0 Downvote
Drew,

Your question why a probe won't work.

When you stand on the floor you do so with two feet. So you have a substantial contact area (=low contact resistance). When you use a probe the contact area is around 1 mm2 at best (=high contact resistance).

Since concrete is not as conductive as a metal is, the contact resistance can be quite high and that is why a measurement with a probe usually gives a false value. Even with a 10 Mohms internal impedance. The main reason for the false reading is not loading of the circuit but hand capacitance. A quick reading can be taken using a wet rag and a metal plate on top on it. Then you can use an ordinary probe to measure voltage between floor and mains. Or resistance between floor and PE.

Gunnar Englund
 
Upvote 0 Downvote
I would also add that a powerstrip breaker trips when the current through the hot lead is greater than 10, 15, 20 amps, whereas a GFCI trips when the _difference_ between the current in the hot lead and the current in the neutral lead is more than a few milliamps. The reason is that the path for that missing current might be through a person with a distressed look on his face and smoke coming out of his ears.
 
Upvote 0 Downvote
ScottyUK,

The one I used in school was probably a two winding transformer and an auto in the same box. I make no claim that they are all isolating, but believe it to be a good idea for this application.
 
Upvote 0 Downvote
Grounding the bench.... The general theory is that a test lead or piece of equipment could make the table live and then become a shock hazard. Of course, the reverse is true. You are also likely to touch a live circuit and with the other arm be leaning on the bench to complete a lethal circuit. A sheet of Formica makes a durable work surface with good insulation. Another reason to ground the bench is to reduce noise pick up. A friend of mine had his shop in a lead lined room that used to be used for radiation work. He could work on sensitive circuits all opened up with no radio station interference.
 
Upvote 0 Downvote
skogsgurra,
Thank you - the resistance thing makes sense now. Also, I wouldn't have dreamed of simply measuring the potential between an outlet and something having a questionable relationship with earth ground - I guess that's probably how my ac voltage supply checks leakage to ground.

geekEE,
That really helped! It was the missing piece; I think I now have the complete picture.

OperaHouse,
I was, at one point, beginning to feel silly for asking about insulation - nice to hear that there are different ways to approach the safety issue.
To all who have begin to worry about me for having these kinds of questions (but still going ahead with setting up a bench!), fear not: I've learned so much in this post, that I've decided to see what other advice is out there and make a general post about safety/efficiency considerations while setting up a bench (ultimately, no matter what I get going, it's bound to be more safe than the mock-up workbench I set up at nite on my kitchen table:) )
Anyway, thanx to all who contributed!



 
Upvote 0 Downvote
The resistivity of concrete in humid environment is expected be in the range of 30 to 100 Ohm-m. To verify the resistivity values, field measures may be required.

One possible alternative is using the 4 point or Wenner's method as describe in the enclose link

 
Upvote 0 Downvote
An interesting dissertation discussing Wenner's method as applied to measurement of permeability of concrete.


As a side note, Wenner's method is the AC version of Van der Pauw's original method described in 1958:



TTFN
 
Upvote 0 Downvote
Status
Not open for further replies.

Part and Inventory Search

Sponsor

Back
Top Bottom