Posted By Garry Homer
Apologies to anyone who may have asked a question that has been lost between threads.

Garry

Posted By Garry Homer
Merv

Your last question looks like it might be the easy one to answer first. Unless you have a need to, do not bother with the Faraday Cage.

Although, thinking about it, this answer requires a definition of ‘need’. I guess it is going to get tough from here!


A starting point with this subject is to find the documentation:

Two documents can be downloaded from the internet; one is Directive 2004/40/EC and the other is the International Commission on Non-Ionising Radiation Guidelines – (ICNIRP guidelines 1998), so Google away.

The ICNIRP guidelines are important as they form the basis for the present ‘action levels’ and ‘limits’ in the Directive. The Directive does not indicate what equipment is involved, nor do the ICNIRP guidelines, however, the guidelines do layout the rationale for the values.

The first clue is the frequency limitation of the Directive, that is 0Hz (DC) and every frequency up to 300 GHz. You might notice the 1998 guidelines you find do not actually cover DC fields but the Directive does. There is another set of guidelines specific only to DC fields. So, although there is the R in ICNIRP, this has nothing to do with ionising radiation at frequencies associated with UV, X-rays etc. The frequency range also does not cover optical, or IR.

That said, the frequency limitation of this Directive is not much of a limitation as it covers every piece of electrical and radio frequency equipment you are surrounded by, in your home/office/workshop/car etc. If these fields had a colour, say blue, we would all see we are living in a blue world. It would also be easy to see where the dark blue areas were and we could all avoid them.

So what shades of blue should be avoided? This is where the Directive and guidelines come in.

The ICNIRP guidelines provide the detail of the permitted amplitude under specific circumstances. Different frequencies couple with our bodies in different ways and so the permitted amplitudes are also different for different frequencies. Extra Low Frequency (say 50Hz) induces currents in our bodies and these effects are taken as being instantaneous (nanoseconds actually), the higher radio frequencies of say mobile phones, broadcast TV, Radio, Radar etc all have a heating effect on our bodies and as this takes time to build up so the guidelines permit time averaged readings. Then there are situations where more than one frequency or source is involved.

These guidelines also distinguish between public and occupational situations, setting much lower values for the public. But, the Directive only tells you what you must do to protect your employees. This Directive has nothing to do with public levels. So we have a situation where the ICNIRP guidelines have a legal status in occupational situations but only a guideline status everywhere else.

The Directive does make some reference to public values, that is, if an employee’s environment is compliant with the public levels and no other risks are involved, then there is nothing else to do other than ensure the situation does not change. This is where every employer wants to be, below the threshold where action and managerial time is involved.

So what equipment or installations are involved? Your DSE, microwave oven and portable phone is a start, but it is the ‘something else’ that is difficult to list because it is so diverse.

The expectation that some employers have that CENELEC will define this list for them, I believe is too optimistic. The pace of invention and bringing new products to market prohibits any such definitive list being possible. There are generic work activities that are already documented where some will exceed the permitted values unless the design of the work changes.

Some domestic appliances exceed both public and occupational guideline values, so some appliances will also go through changes.

Find the two documents mentioned and I will get back to you with a few examples.


Garry Homer

Posted By Garry Homer
Careful John

The existing NRPB recommendation, as stated by them in 2004 is that the ICNIRP guidelines should apply in the UK. Therefore the NRPB (HPA) strictly have no current guidelines of their own. They had fought against the concept that ICNIRP used of having 2 levels, one for the public and one for occupational situations. You can have a silly situation where you can use a hand drill or similar at work but not at home! Also the factor between the public and occupational levels was not scientifically established, but 5 seemed a good number to use.

Once Directive 2004/40/EC was approved and to be applied in all member states, it looked like it did not leave the NRPB any further room to manoeuvre.

It is therefore important that anyone with a copy of the NRPB yellow books for the frequencies involved must get a copy of the ICNIRP guidelines ASAP.

As the NRPB guidelines were just that, to my knowledge there has not been any enforcement of them as they do not have any legal status. The HSE has waited a long time for something to come along that will enable them to get involved.

The 'if you are complying' is the big issue. I do not believe companies and the HSE know how far and wide this Directive will apply. The HSE consultation so far has not been one that provides information to the wider audience. The business side of meetings with the HSE seem to show they are worried about where they will find the competent people who can carry out the assessments. Businesses will find companies offering their services, wandering through their buildings carrying some sort of meter, but having no idea what they are doing. The Directive states that only competent people shall carry out the assessments, but there are not many about.

I know some companies are keeping their heads down at the moment as the necessary investment could be significant and they do not want to do it until they must. In 2004 I made my clients aware of the changes from NRPB to the ICNIRP guideline levels. (A reduction by more than 3 times at 50Hz for occupational situations. A 16 fold reduction in public situations) I have also raised the issue with some that future legally required compliance with the Directive is in doubt.

Most companies are not clued up. I have surveyed the roof tops for some companies because their staff have had concerns about the mobile phone base station on the roof, but their big issue has been some other situation I have noticed in passing through their buildings and to which they have not given much thought.

Some companies have work situations that do not comply with the higher NRPB guideline levels, never mind the ICNIRP guidelines. This must show the lack of knowledge the Trade Union reps have.


As a taster for where compliance issues can be:

Shops; some anti-theft security loops in doorways are very close to exceeding the guidelines, some may.

Offices; parts of switchrooms if their load exceeds around 300 - 500 KVA.

Industry; resistance (spot) welders, induction furnaces, substations, switchrooms, busbar systems, RF dryers, etc, etc.


Establishing compliance can be tough, but it is easy compared to finding solutions!


Garry Homer

Posted By GT
Garry,Merv
Same subject,where were you? see similar thread of 25.02.07

Limited interest and responses.

GT

Posted By Garry Homer
Thank you for your opinion Jay.

I believe the paper you mention does little more than reveal a reluctance to accept the inevitable.

Whatever fine words come from tomorrow’s meeting, the fact remains there are many jobs in industry where the fields exceed the levels set out in the Directive. Some even exceed the older and higher NRPB guidelines.

See:

NRPB R265 'Review of Occupational Exposure to Optical Radiation and Electric and Magnetic Fields with Regard to the Proposed CEC Physical Agents Directive'

This may not be available to download from the HPA website but is available to order: ISBN 0 85951 368 8

This is a 1993 document that carried out an appraisal of the proposed Physical Agents Directive as it was then, Annex 5 I believe. This new Directive is much simpler.

The intention is not to worry anyone. This is a wake up call for those who will be asked to say what they have done to show how they comply. Fail to prepare, etc, etc.

The HSE already know who to target first.

I would like to see H&S professionals on top of this subject, if it is relevant to their business. I do not want to see more so called experts crawl out of the woodwork come 30 April next year who do not have a clue as to what they are doing. When companies finally come to mine for assistance or second opinion, the lack of understanding shown by their expert is very revealing!

One could hope the Directive and the output from CENELEC that will form a COP will stop them, but I doubt it.


Garry Homer

Posted By Garry Homer
There should be no requirement to check either and especially the (CE marked?) LCD monitor.

Yours is a common problem that comes my way every now and then. The problem is an example of the hysteria and bad science that is around and that some others trade on.

The big problem is people do not know what radiation means and how very common it is. We could not see without it!

The big difference is the split between ionising and non-ionising bands. Non-ionising radiation does not have enough energy to damage our DNA, unlike x-rays etc.

When CRT (Cathode Ray Tube) monitors were last checked, the very low level of ionising radiation detected at the front of the monitors was found to be more to do with the origin of the sand used in making the glass than the process of producing the image!

If anyone wants to know more about CRT monitors, as a starting point see:

Documents of the NRPB, Health Effects Related to the use of Visual Display Units, Volume 5 No2 1994. ISBN: 0 85951 376 9

There will always be some people who believe if something is regulated, then there must be a problem at all levels, not just at, or above the set limits. There are plenty of stories in the Press and on TV to keep feeding this underlying distrust of those who make the decisions on what is safe.


Garry Homer

Posted By Garry Homer
GT


The 1998 ICNIRP document ‘Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz)’ that form the basis for the bulk of the values in the Directive is short on detail when it comes to DC (0Hz).

More detail is provided the 1994 ICNIRP document ‘Guidelines on Limits of Exposure to Static Magnetic Fields’ that I assume will be the source of the detail for the Directive’s implementation. An assumption that could be wrong, but it would be strange if the reliance upon the work of ICNIRP stopped short of using this DC specific document.


The differences are:

The 1998 document is for time varying fields and their lowest category is 0 to 1Hz, whereas the 1994 document is specific to DC or 0Hz only. To include a little history, the NRPB guidelines used to include DC in their ‘Restrictions on Exposure to Static and Time Varying Electromagnetic Fields’ and used categories of ‘Below 0.4 Hz’ and ‘0 – 1 Hz’.

What happened prior to the Directive appearing? Well the previous guideline details were as follows:


ICNIRP 1998 ‘time varying document’ occupational Reference level, up to 1 Hz is 200,000 microtesla (0.2T)

This is taken to be an instantaneous value as there is no reference to any permitted time-averaging in the document. Normal practice in the writing of these guidelines is to state the values are is to taken as instantaneous values unless time-averaging is specifically mentioned.


ICNIRP 1998 occupational Basic Restriction is 40 milli-Amperes / m*m. The maximum current density in Head and Trunk.


ICNIRP 1994 occupational Limits of Exposure for Static Magnetic Fields is 200,000 microtesla (0.2T) and this is the whole working day time weighted average with a 2,000,000 microtesla (2T) Ceiling value.

Also 5,000,000 microtesla (5T) for Limbs.


NRPB 1993 Investigation Level – (No distinction between public or occupational situations) for frequencies less than 0.4 Hz is 200,000 microtesla (0.2T)

This is taken to be an instantaneous value as there is no reference to any permitted time averaging in the document.


NRPB 1993 Basic Restrictions – for tissues of the head neck and trunk for 0 – 1 Hz is 200,000 microtesla (0.2T). This is a 24 hour average.

Once more with a 2,000,000 microtesla (2T) Maximum value and also 100 milli-Amperes / m*m for the maximum induced current density.

Their Basic Restriction for Limbs was 5,000,000 microtesla (5T).


From this background, perhaps we should have expected more of the Directive that simply states:

The Action Value, 0 – 1 Hz, is 200,000 microtesla

The Limit Value, 0 – 1 Hz, is a current density for the head and trunk of 40 milli-Amperes/m*m.

The lack of detail about DC in the Directive is causing a bit of a dilemma for the operators of MRI equipment at the moment. Looking at the broad consensus of the values across the three guidelines above highlights the Directive misses the point about time averaging and the higher permitted values for limbs.


A quick word on units.

I always work in units of magnetic flux density, a natural progression from an electrical engineering background where the ‘old school’ method was to work in units of Weber/m*m.

1 Wb/m*m = 1 Tesla.

Magnetic Field Strength is sometimes quoted in terms of Amperes/meter. The conversion approximates to 1 A/m = 1.25 microtesla.

For those who like Gauss; I microtesla = 10 milligauss

A Tesla is a large unit so the common usage is usually microtesla or sometimes millitesla. My favourite is the microtesla as it puts things into perspective for me. This might seem a little odd at the moment as all the previous values I have quoted involve many noughts, but this may become clear soon.


Your work situation:

You describe metallic objects defying gravity in the DC magnetic field. If these objects are not becoming airborne, ie flying across the workspace and attaching themselves to the source of the magnetic field, then this indicates the magnetic flux density is within the guidelines as the threshold for the such movement of ferromagnetic materials is taken as 3,000 microtesla, ie 1.5% of the 200,000 microtesla Action Value contained in the Directive.

If you are describing how objects stand on a surface of a magnetic field source, then the rule of thumb of 3,000 microtesla may not apply. But then perhaps your neck and trunk is not in close contact with the source either. The Directive in its present form does not give any clues as to what do about your hands!


Next important bit – The DC Current Source

When it comes to the assessment of DC fields it is important to understand how the DC current source is being created. The usual method is to rectify the AC supply. If this is the case in your workplace then consideration has to be given to the presence of any ‘ripples’ on the DC waveform. These ripples are a consequence of rectification and exhibit the frequency characteristics of the primary AC source. This is basically where a proportionately very small AC current is imposed on the perfect straight line of a true DC waveform. If you are using a DC generator, their may be similar frequencies present due to the pick-up off the commutator.

Why the interest in these frequencies? These must be taken account of because the guidelines, and hence the Directive, set much lower values for AC magnetic fields as shown below. (Values have been rounded)

Directive Action Values for 50Hz magnetic fields and its harmonics.

50Hz - 500 microtesla
100Hz - 250
150Hz - 167
200Hz - 125
250Hz - 100
300Hz - 83
350Hz - 71
400Hz - 63
450Hz - 56
500Hz - 50
550Hz - 45
600Hz - 42
650Hz - 38
700Hz - 36
750Hz - 33
800Hz - 31
820 to 2,500Hz - 30.7 to be pecise.


ICNIRP public reference values are 1/5th of those shown above.

So, by comparison with DC fields, the permitted AC magnetic field is small and this is where the units of microtesla fit in more readily.

In industrial applications where DC is being used that is sourced by rectification of the AC supply, it is usual to find the limiting factor on how close a person can be to heavy current conductors etc, is more to do with the AC ‘ripple’ components of DC waveform, than simply the DC field.

Another point to consider is that low frequency magnetic fields like those detailed above are set to limit the value of induced currents in our bodies. At these frequencies, it is to limit currents interfering with the normal operation of our central nervous systems, hence the reference to head, neck and trunk where the CNS is concentrated.

Movement in a DC field also induces currents in our bodies, so these two effects have to be considered in combination. For example; if the DC field was found to be 50% of the guideline value, this would only leave 50% of the AC guideline value available for the AC field before the combined effect reached the Directive’s field value. If the AC field contained harmonics, then the remaining 50% would also have to be shared out between the fundamental and harmonic currents in accordance with the limiting value set for each individual frequency involved.

And I’m often asked, can I just walk around with a meter to check if it’s ok!

Analysis becomes very complex when you look at pulsed fields, or, where DC fields are reversed very quickly.



Garry Homer

Posted By Garry Homer
Tony

You need a seminar designed especially for the type of workshops you operate. Or better still, you need one-to-one training so that a member of your staff can decipher the readings you will collect once you have decided what instrument to use.

Unfortunately neither of these options are readily available. Those seminars that do exist are not deep into parts of the subject that you need and tend to be broad and shallow. This is probably due to there being little interest at the moment so the seminars are as broad as possible to attract enough attendees to make the seminar financially justifiable.

I know one organisation that claims to be able to design training of the sort you need but my contact with them causes me some concern as to there true abilities. For obvious reasons I cannot name and shame on here.

The choice of instrument is not a simple one. There will not be one instrument that can cope with the range of fields you will encounter and the speed at which you will need to collect the data.

For example, with spot welders you have only a few seconds duration at maximum current.

Therefore, instruments that claim to be able to make the assessment simple and give you a direct read-out showing the percentage of the ICNIRP occupational value, do there job well in almost all applications, but they might not be able to cope with a short burst of mag field. This is because of the time the more sophisticated instrument takes to analyse the values it is measuring.

The suppliers I have quizzed have not been able to say they will definitely work properly in these circumstances. This is a pity as it could make my like a great deal easier!

The output from CENELEC could be enlightening as they may specify the instrument characteristics that are required for certain situations and then manufacturers may respond.

If you are thinking of buying any instruments, it may be useful to get the sales engineers to bring their instruments to your workshops and give you a demonstration of how they work.

The downside to this is the values may indicate that you are exceeding the set values and your production staff my get very concerned.

With spot welders, where a person holds the metal to be welded, because they are so close to the heavy current electrodes the magnetic fields can be high. You should expect them to be high and any instrument that shows otherwise should be treated with caution.

The work by the NRPB as it was, on spot welders, detailed a 16 KVA welder that created values greater than the more easily measurable Reference Levels (ICNIRP)/Action Values (Directive). When they reported their interpretation of the Basic Restriction using body currents, they used phrases like 'unlikely to' etc which may not satisfy any union rep who will prefer a straight yes/no answer to compliance.

There are many documents freely available to be downloaded at the HPA website that could wet the appetite of your electrical engineer.

Regards

Garry