Static Magnetic Fields Safety hints for entering magnetic

Static Magnetic Fields Safety hints for entering magnetic field volumina Fritz Szoncso SC/GS

What is a Static Magnetic Field? A magnetic field is the area around a magnet or electrical current that has a detectable magnetic force. “Static field” indicates that the field does not change in value or polarity over a period of a few hundred seconds. Therefore no induction can occur onto still objects in the field.

Generation of Static Magnetic Fields Static or quasi-static magnetic fields are generated by: Electric coils, such as magnets Permanent magnets in loudspeakers, small motors, horse-shoe Superconducting magnets for LHC and various experiments DC-wiring Klystrons, radio and microwave tubes Electromagnetic lifting devices Nuclear Magnetic Resonance (MRI) scanners

Properties of Static Magnetic Fields A magnetic field may be described by: a) Magnetic flux Weber, or Wb, or Voltseconds b) Magnetic flux density Tesla, or T, or Voltseconds per square meter (also Gauss in old literature with 1 T=10000 G), c) Permeability µo= 4 π 10 -7 Asec/Vm

Properties of Static Magnetic Fields Field penetration, deviation, configuration a) Static magnetic fields penetrate all materials, including the human body. b) Magnetic fields cannot be shielded. However, the flux can be deviated and concentrated by putting higher permeability materials (iron, nickel alloys etc. ) into the field path. c) Stray fields can never be entirely eliminated, except by active field cancellation. d) Magnetic field lines circle around the magnetic poles but never touch them. The lines have no beginning and no end.

Impact of Static Magnetic Fields Static or quasi-static magnetic fields can impact the industrial environment: a) Forces exist between the coil(s) and all surrounding ferromagnetic objects b) Many things do not work any more when a magnetic field is present: Circuit breakers, microphones, fans, transformers, etc. c) Magnetic fields store energy. Transferring and storing this energy is a technical risk.

Hazards of Static Magnetic Fields The static magnetic field can a) Interfere with medical implants and devices b) Move, attract, accelerate, tilt ferromagnetic devices (hammer, drill, keys, etc. ) c) Cause physical symptoms like nausea and vertigo d) Erase data on magnetic memory material (credit and access card) e) Induce voltages and exercise forces in apparatus moved through the field (electronic devices, but also safety shoes)

Issues around Static Magnetic Fields What can I do to be safe ? By knowing about the risks I can adapt my behaviour for maximum safety. I make an effort not to approach ferromagnetic objects. I know about forces that can hinder the movements of (normal) safety shoes. I have a precise field map.

Static Magnetic Field Comparisons For comparison: Earth field: 0. 00006 T Pacemaker limit: max. 0. 0005 T Limit for General Public: max. 0. 01 T Exposure during 8 h workday: max. 0. 2 T For comparison: LHC inside magnets 8. 33 T CMS inside magnet 4 T ATLAS inside strongest magnet 2 T

Issues around Static Magnetic Fields ALARA principle Keep it short. ALARA=As Low As Reasonably¨ Achievable Supervisors have the obligation to plan work for shortest exposure times. Nobody remains exposed to fields longer than absolutely necessary.

Practical hints for ferromagn. objects Additional hazards Hazard: Loose Forces exist between all ferromagnetic objects may fly. Or turn. Risk: You could be in the way. Reality: Several incidents already happened at CERN, e. g. powertool projected into magnet aperture at high speed (ISOLDE), scaffolding flipped over (ALICE), entire test detector destroyed (CMS H 2)

Equipment in Static Magnetic Fields Equipment in the field All magnetic cards (credit card, access card, etc. ) are affected by the magnetic field. (boxes are provided for safeguarding your valuables) Equipment using conventional loudspeakers does not work (walkie talkie, intercom) Conventional computer screens do not work any more. Even small stray fields have visible effects.

Medical requirements Exposures beyond 10 m. T are to be recorded indicating values, start and end times. Persons who are not physically fit, in particular persons with blood circulation problems, will not be allowed in. The values between 10 m. T and 200 m. T are sufficiently far from any dangerous level.

Apparatus in Static Magnetic Fields Sensitive apparatus Sensitive equipment can be destroyed by induced voltages. Move equipment at lowest possible speed, do not make jerky movements. When cables are attached the surface affected by the magnetic flux can be large - Induced voltages are then much higher. Equipment exposed to vibrations will be suffering from induction at the vibration frequency.

Transient Fields and Quench Transient field (includes quench) In principle a field change induces voltages in the entire field volume. There is no danger for persons because the voltages induced are very small. We do not foresee additional precautions.

Issues around Static Magnetic Fields Forces on ferromagnetic objects The forces may be surprisingly large. No loose ferromagnetic objects are allowed in field volume. No unbolting is allowed inside the field volume. Tools brought into the field volume must be compatible with work in a magnetic field volume. Power tools are forbidden inside the field volume. They will not work anyway.

Issues around Static Magnetic Fields Forces on ferromagnetic objects The forces may provoke a torque. Ferromagnetic objects want to align themselves with the field lines. In case you find loose objects inside the field volume PLEASE DO NOT PICK THEM UP. The objects must be removed once the field is off again. There are non-magnetic safety shoes. This type should be acquired for persons entering the field volume.

Issues around Static Magnetic Fields Metallic structures in the field All iron structures in the field will carry a larger magnetic flux than the surrounding air. It is forbidden to be close to large ferromagnetic objects. The iron floors will put some force onto normal safety shoes. BE CAREFUL - there is a feeling like sticking to the surface. Metallic floor pieces (Caillebotie) must be bolted and not just lying there.

Issues around Static Magnetic Fields Similar to our case: MRI

Issues around Static Magnetic Fields Conclusion THINK prior to entering an area at risk. REMOVE all items that react on magnetism. MOVE SLOWLY inside the field volume, in particular when crossing field lines. RESPECT Code A 6 - the two person rule of working, and have a corresponding person outside the field volume for any safety action you might require. Exercise care. Rescuers have difficult access and need to able to switch off the field prior to any intervention. Check on a field map to what you would be exposed at which spot.