Safety	Van de Graaff generator hazards
Intro        Physiological effects    Van de Graaff shocks    Operational rules
Excepting to persons with certain medical conditions, there is little risk of harm from electric shock from a Van de Graaff generator whose dome does not exceed 25 cm. Two cases of shock are analysed - a direct spark to the body from a charged dome, and an unintended discharge of a charged person. The first of these leads to a limit for maximum dome size. The article finishes with guidance on how to run the machine safely. Introduction We discuss firstly the risk of harm to pupils with unusual medical conditions,, based on correspondence with a medical inspector of the Health and Safety Executive  Listed below are cardiac conditions which may place someone at increased risk of ventricular fibrillation. This is an An uncoordinated, rapid, electrical activity of the heart; there is no effective pulse and death ensues rapidly.  Subjects with any of these conditions should not receive electrical discharges from this equipment: coronary heart disease (e.g. angina, history of heart attack) cardiac rhythm disorders intra-cardiac conduction pathway anomalies presence of an implanted cardiac pacemaker hypertension (high blood pressure) Regarding epileptic subjects, the risk of seizure is unlikely, but epileptic subjects should not be allowed to take that chance. There is no evidence to suggest that persons without  any such medical condition are at risk of ventricular fibrillation on getting electric shocks from a small Van de Graaff generator.  This is underpinned by a theoretical understanding, which indicates no significant risk of harm; unless there is an electrical fault condition in the apparatus itself.   Figure 1 The Frederiksen Van de Graaff generator (product code 3700.50) (suppliers include DJB, Nicholl, PASCO (SF‑9722) and Scientific & Chemical (XES 080 010)). Having a 22 cm diameter dome, this machine should be incapable of causing a severe shock.

Physiological effects The physiological effects of electrostatic discharges [1] depend on the spark energy (Table 1).  This, in turn,  is depen-dent on the capacitance of the system and the stored charge, or potential with respect to earth. The capacitance of the human body lies between 100 and 300 pF. Spark energy (mJ) Physiological effects of electrostatic discharges 1 Smallest spark energy felt 10 Some find 10 mJ uncomfortable due to muscular contraction. Others can accept several hundred millijoules before experiencing sharp muscular contraction 1000 Affects everybody severely There is an accident history of people being rendered unconscious by discharges of several joules Table 1  Effects of electric shock as quantified by spark energy. If the charging voltage is greater than 1 kV, which it is with a Van de Graaff, then the thresholds of perception and pain can be related to the capacitor discharge.  Respective values would seem to be around 0.5 mC and 8 mC [2, 3] by extrapolating from a graph in the relevant British Standard.

Van de Graaff shocks If the field strength between two flat or large radius conducting surfaces exceeds 3 x 103 kV m-1 then a spark occurs [1].  Related to this [4], the theoretical maximum potential that the dome of a Van de Graaff can reach is 3a  x 106 volts, where a is the radius of the dome in metres.  Some values are shown in Table 2.  The capacitance of the dome and stored electrical energy are derived using C  =  4pe0a and E  =  1/2CV 2. Dome diameter (cm) Capacitance (pF) Maximum potential (kV) Maximum stored energy (mJ) 20 11 300 500 25 14 375 980 28 16 420 1370 Table 2  Electrical properties related to dome diameter. It should be appreciated that the values for stored energy and potential will seldom be attained.  They may hold when the air’s relative humidity is abnormally low, such as sometimes occurs after the passage of a cold front, or in a fohn wind.  In other words, these tabulated values are the highest a machine can reach.  On a typical day, values will be lower; the machine will be much safer. There are two ways whereby the user can get a shock – from coming too near to a charged dome, or from an unintended discharge while being charged deliberately. A person will get an unintended shock by carelessly coming too close to the charged dome.  Most of the stored energy on the dome may then discharge to earth through the person’s body giving the unintended shock.  If we set a limit on the stored energy that should reside on the dome to be 1 J, then the maximum dome diameter is 25 cm.  There is, so far as we are aware, just one product on the educational market that breaks this limit.  It is the newly designed STE model with a dome diameter of 28 cm.  (We have raised this with STE.) Another means of shock occurs when a person - usually a pupil – is deliberately charged up and gets unintentionally discharged.  In such a system, the electrical properties of the human body play a significant part.  The highest potential reached is governed by the minimum radius of external body parts. The value might be 5 mm with a pinkie.  The system voltage comprising dome and body might then reach V = 3a x 106 V  = 15 kV.  With the body’s capacitance of 300 pF, the energy to be discharged = 1/2CV 2 = 34 mJ and the charge stored on the person = CV = 4.5 mC.  A sudden discharge of this amount of energy and charge would certainly be noticed.  It would probably be disagreeable.  It might even verge on being painful, but is unlikely to have any other direct effect. Do bear in mind that any person getting a shock is at risk of harm from jerking or falling over in fright.  There is then an indirect risk of a blow to the head, or damage to muscles, bones, or other parts. Figure 2 The STE Van de Graaff generator (product code 10178) (dealers include Anderson Scientific (10178), Griffin (XJE-350-V) and Scientific & Chemical (XES 030 010)). Having a 28 cm diameter dome, this machine may be capable of causing a severe shock.

Operational rules In consideration of the above, the following rules should be applied: Beforehand the teacher should check the appropriate records held at school for any relevant heart condition. Before using the equipment, the teacher should warn the class that it should not be used by anyone with such a heart condition. Charge only one person at a time to limit the charged capacitance of the system.  The severity of the electric shock increases with capacitance.  The shock can increase with the number of persons being charged simultaneously. Persons being charged should be limited to volunteers.  It is generally inadvisable to attempt to charge everyone in a class, whether singly, or together in a chain (as explained above). Because a human body has capacitance, do not let someone touch a charged dome, then walk away.  In these circumstances such a person may carry quite a lot of charge and experience a disagreeable and possibly unexpected electric shock on touching earth.  Any other person touching such a charged person is also at risk of getting a shock. The dome may be safely discharged by touching it with an earthed, metal conductor mounted on an insulated stand or handle.  If this is operated properly, the experimenter should not receive a shock. The dome should be discharged immediately after every operation ensuring that it never stands idle in a charged condition. The dome may be safely discharged through the human body by arranging that the person getting the discharge is in poor contact with an earthed conductor.  Instruct him to place one hand, palm down, firmly in contact with the benchtop, which is presumed to be wooden, or of similar, low conductivity.  The dome may then be discharged by bringing the other hand up to it, either by direct skin contact with the dome, or through a hand held metal wand or sphere.  Break contact with the dome before breaking contact with the bench.  These directions assume that the benchtop insulation resistance is at least one megohm to earth, which is usually the case.  By following these instructions the dome should be safely and completely discharged with the person experiencing only a slight physiological effect. The demonstration with hair standing on end may be done safely by adapting the above procedure.  Begin with the generator off and dome uncharged.  Instruct the pupil to stand on an insulated platform (such as a plastic basin) and place one hand on the dome while ensuring that no part of the body or clothing is in contact with the bench, or another pupil.  Start the generator and run until hairs stand on end.  Stop the generator and instruct the pupil to place his free hand on the wooden benchtop while still keeping his other hand firmly on the dome.  Wait several seconds until completely discharged.  Remove hand from dome, step off the platform and walk away from the apparatus.  The dome at this stage will not be carrying a charge and will be safe for another person to touch. The demonstration of lighting a Bunsen flame by discharging through a human body to the Bunsen funnel would seem also to be fairly harmless provided that the Bunsen funnel does not have a low resistance path to earth, which is usually the case.  Test lighting the Bunsen with an insulated lead.  If the air discharge path is small and the accompanying sound is slight, it should be safe to use a human body for the discharge path. If the capacitance of the system were to be greatly increased, for instance by connecting the dome to a Leyden jar, the stored electric energy can increase to a dangerous extent. All science staff should be trained in how to work with the Van de Graaff generator, being made aware to avoid a direct path through the human body to a good earth (an earthed conductor of low resistance). References BS 5958: Part 1 : 1991  Code of practice for control of undesirable static electricity  Part 1  General considerations  BSI. PD 6519 : Part 2 : 1995  Guide to effects of current on human beings and livestock  Part 2  Special aspects relating to human beings  BSI. Preventing electric shock  Bulletin 173  SSERC  1992. Berg R.E.  The Physics Teacher  28  5  (May 1990)  pp 281-5.

SSERC Visitors