EMF from concrete slab heating

[jamesmann]

I have a client in Ottawa that owns two large 250 suite apartment buildings with a poorly installed concrete slab electric heating systems. These buildings are 25 years old and suffer from excessive EMF in many areas of the building causing TV and computer problems.

The heating cables are unusual in that they are continuous over an entire floor-plate of the building and draw about 50 amps at 600 volts. Usually we see these systems installed on a "suite-by suite" basis with cables zoned for each suite.

Recently, we have installed SCR controls on the heating cables to provide improved control but this has not improved the EMF problem...(I don't think it has made it any worse either - even with the increased harmonics)

We are looking for an inexpensive way to measure the EMF field (in order to move TV's, etc.) and/or other solutions. Will it work if we put a piece of thin steel on the floor (under a TV) with a wire to ground?

Any help on this issue would be appreciated.

Regards,
James H. Mann, P. Eng.
Consulting Engineer
Mann Engineering Ltd.
james@mannengineering.com
www.mannengineering.com
(416) 201 9109 office
(416) 201 9122 fax

 

[big john]

Re: EMF from concrete slab heating

James,

Covering the floor in grounded steel would eliminate the electric field of the interference you're experiencing. Doing that may not improve the operation of your television and computer equipment, as you would still have the magnetic field to contend with which at high enough strengths would cause screen distortion and the like.

-John

 

[pwhite]

Re: EMF from concrete slab heating

james,
i did an installation with scr controls and ended up replacing them with mercury displacement relays. on our heater application, one leg of the heater system was always energized.

you can get several gaussmeters. each meter will have its own magnetic sensivity. i'm sorry, but i don't remember the brand name. i use mine in an industrial setting to determine the field strength before doing maintenance reqiiring a laptop computer. it cost $ 35.

 

[ed downey]

Re: EMF from concrete slab heating

James,

I Use The F.W. Bell Model 4080 Triaxial Gaussmeter. It Works Great For A Determination Of The General Field Levels. If You Want To Pinpoint Exactly The Magnetic Field You Might Have Better Luck With A Single Axis Gaussmeter.

Also Karl Riley's Book "Tracing EMFs In Building Wiring And Grounding"
-Ed

 

[brian john]

Re: EMF from concrete slab heating

James as mentioned there are a variety of gauss meters available at a reasonable price, check Jensen Tools.

As for randomnly placing steel on the floor this should NOT be done intill such time as you have consulted with someone fimilar with shielding. A proper shield costing LOTS can contain the EMF from one side but are there bottom apartments.

Is the PC/TV problem there when the heat is off?
If so check for other sources of ground current. Start by measuring the current on the Main Bonding Jumper at the main service and at seperatly derived systems.
Check for Ground current at the main ground electrode (espically water pipe).
Do Zero Sequence readings at each distribution panel. Measure all the current utliizing a LEM/AEMC flexable CT that is the phase and neutral (encompass all), reading should be zero (0) amps are something very close to 0.

 

[Guest]

Re: EMF from concrete slab heating

Here's a link to some gaussmeters:

http://www.magneticsciences.com/GaussmetersAndAccessories.html

 

[karl riley]

Re: EMF from concrete slab heating

Yes, the Bell 4080 is accurate and is compensated for harmonics. A $35 meter is almost guaranteed to be inaccurate (I have lab tested many gaussmeters) and may read excessively high if there are harmonics present.

First determine if the field is from net current. If so, no passive shielding will have any effect. Net current is identified by the fact that the field weakens directly with distance. Double the distance from the source and you will read half the field strength. Net current would either mean a correctible wiring error/s or else the heating circuit does not have the neutral running with the hot.

So first determine if it is net current. Your $35 meter should be able to do this, even if the values shown are not true. If by doubling the distance to the source cuts the field to 1/4, then it is a shieldable field (not net). Then try the Amuneal website (a good shielding company website with lots of info).

Karl

 

[jamesmann]

Re: EMF from concrete slab heating

Thanks to everyone!

This problem only exists during "heating" periods and has been an issue in this building since it was built.. I just bought a cheap $50 gaussmeter to see if we can move TVs to lower EMF areas of the apartments. I understand that this may not be accurate but it may be OK for this purpose.

This is a 3-phase 600volt system with no neutral in the building riser..each floor of the building has one phase looped through the slab in 12 suites consuming about 50 amps max.

Other electrically heated buildings that I have seen use lower amps as the loops are much smaller (i.e. one suite only)

As such, I believe that EMF issue will be a result of net current only. I think that the original installation was looped improperly or just not thought out..

It would appear from your comments that there is no practical shielding solution and it is impossible to consider any changes to the heating system due to the massive costs invloved.

As such, my idea is to move the TVs/PCs with my cheap meter....any other ideas??

i.e. I can change the heating controls from 0-100% to a on-off duty cycle rate that will give short periods of no EMF.

james mann

 

[karl riley]

Re: EMF from concrete slab heating

Yes, if the heating circuit is a continuous snaking of the hot element, without looping back on itself, it is uncorrectable. Electric blankets used to be wired like this, but the EMF health issues caused the companies to wire them with the hot running back like a switch leg, thus basically cancelling the field.

Are you talking about moving TVs and computers to areas of less field, while giving the OK for the humans to be placed anywhere? You may be in for quite a liability situation if the EMF health effects evidence triggers the lawyers to sue again. All you need is one employee or resident who developes cancer.

I have seen offices vacated because of EMF levels until the levels were brought down. Just the possibility of health effects causes prudent companies to get the problem fixed.

Installation of another form of heating may be the only solution.

Karl