Floating Circuits - Protection against electric shock

Transcription

1 Floating Circuits - Protection against electric shock Introduction TECHNICALLY SPEAKING by Richard Nute 1998 Richard Nute FLOATING CIRCUITS PROTECTION AGAINST ELECTRIC SHOCK Probably the least-understood issue in product safety is that of protection against electric shock from a floating or isolated circuit A floating or isolated circuit is a one that has no connection to any other circuit or to ground For the purpose of this paper, we assume that the pole-to-pole voltage of the floating circuit is hazardous voltage Such circuits are commonly used as high voltage supply circuits for the screen backlight for laptop computers Figure 1 depicts a floating or isolated circuit The energy source is grounded For the purposes of this discussion, the secondary circuit (pole-to-opposite pole) is Hazardous Voltage (as defined in IEC 950, Subclause 1283) and hazardous current (exceeds the limits for Limited Current as defined in IEC 950, Sub-clauses 1286 and 242) The secondary circuit is isolated from all other circuits by the transformer functional insulation, and the other functional insulations However, there is a very small leakage current in the stray impedances of the functional insulations Typically, this current would be in the microampere range, but may be higher if the circuit frequency is in the kilohertz range One characteristic of a floating or isolated circuit is that the circuit, from either pole to ground (or from either pole to any other conductive part), under normal conditions, is a Limited Current circuit as defined in IEC 950, Sub-clauses 1286 and 242 Another characteristic of the floating or isolated circuit is that it has TWO simultaneous shock current paths, one being pole-to-opposite pole, and the other being pole-to-ground-to-opposite pole Figures 2A and 2B show the shock current path for the two situations (For the purposes of this paper, each shock current path is discussed separately) M:/ /Floating Circuits - Protection ag 1/6

2 Pole-to-opposite pole Figure 2A depicts the circuit path when a man simultaneously touches both poles of the floating or isolated circuit In this situation, there is no insulation The current is limited only by the impedance of the body of the circuit and the man, then we prevent shock current through the body See Figure 3 If we interpose Basic Insulation between one pole Notice that the opposite pole is still accessible But, the Basic Insulation prevents shock current through the body,,, We must also consider the failure of Basic Insulation In the case of failure of the Basic Insulation, shock current would pass through the body as if there was no M:/ /Floating Circuits - Protection ag 2/6

3 insulation We have several options to prevent electric shock current We could interpose a second insulation on top of the Basic Insulation, thus making a Double Insulation system Or, we could interpose a grounded metal part on top of the Basic Insulation to create an equipotential environment Note that one pole of the floating circuit must have Double or Reinforced Insulation between it and ground, while the other pole need have only functional/operational insulation (This follows because we originally depicted the man as touching one pole of the circuit as if it was a NORMAL condition) (Note also that failure of the functional/operational insulation actually shunts the current AWAY from the man!) Normally, however, the man would not have access to either pole of the floating circuit We can prevent pole-to-opposite pole current by preventing simultaneous access to BOTH poles of the floating circuit If we interpose an insulating barrier between one pole of the floating circuit and the man, then we can define that barrier as Basic Insulation In the event of failure of that Basic Insulation, there is no electric shock current in the man Insulation (because it provides insulation against the SECOND body connection) If we extend that same insulation such that it is interposed between the OPPOSITE pole and the man, then we can define the OPPOSITE pole portion of the insulation as Supplementary Voila! With a single layer of insulation, we have created an elegant and least-cost Double Insulation scheme The same insulation one part -- plays both the role of Basic Insulation and Supplementary Insulation! See Figure 4 Pole-to-ground-to-opposite pole Figure 2B depicts the circuit path when a man simultaneously touches one pole of the floating or isolated circuit and ground In this circuit, functional/operational insulation is interposed between ground and the opposite pole of the isolated/floating circuit The current is limited by the impedance of the functional/operational insulation This being the case, then the functional/operational insulation must be Basic Insulation See Figure 5 Let s now examine the situation for the M:/ /Floating Circuits - Protection ag 3/6

4 event of failure of the Basic Insulation If we short-circuit the Basic Insulation, we then have no current limit, and an electric shock current passes through the man We have several options to prevent electric shock current We could interpose a second insulation adjacent to the Basic Insulation, thus making a Double Insulation system Or, we could interpose a grounded metal part between the Basic Insulation and ground to create an equipotential environment (Note that failure of the functional/operational insulation actually shunts the current AWAY from the man!) This presents the same interesting construction as for the pole-to-opposite pole case One pole of the floating circuit must have Double or Reinforced Insulation between it and ground, while the other pole need have only functional/operational insulation (This follows because we have depicted the man as touching one pole of the circuit as if it was a NORMAL condition) Normally, however, the man would not have access to either pole of the floating circuit Assume that neither pole is accessible In the event of failure of the pole-to-ground Basic Insulation, we can prevent shock current by preventing access to either pole of the floating circuit Since we have defined the pole-to-ground insulation as Basic Insulation, we can define the pole-to-man insulation (barrier) as Supplementary Insulation The most elegant and least-cost solution is to interpose Supplementary Insulation between the circuit and the man See Figure 6 Insulation for protection against electric shock If we combine the results of our two situations, pole-toopposite pole and pole-to-ground-toopposite pole, we find that interposing Basic Insulation both between the floating/isolated circuit and accessible conductive parts and between the floating/isolated circuit and ground provides both Basic and Supplementary insulation in a single insulation See Figure 7 A conductive barrier for protection against electric shock M:/ /Floating Circuits - Protection ag 4/6

5 If we replace the insulating barrier with a conductive barrier, the insulation requirements change See Figure 8 There is no current path to the man There is no current path to ground There is no leakage current There is no Basic Insulation There is no Supplementary Insulation In this configuration, there is no current path for electric shock even though the conductive barrier is floating! This is an important concept Other secondary circuits may play the role of the conductive barrier If those circuits adjacent to the floating circuit are relatively low impedance, then functional/operational insulation is sufficient, and there is no need for Basic or Supplementary insulation between the floating circuit and other secondary circuits! M:/ /Floating Circuits - Protection ag 5/6

6 Conclusion We have shown two simple, cost-effective schemes for protection against electric shock from a floating or isolated circuit: 1 Preventing access to all parts of the circuit by means of a barrier of Basic Insulation, either air insulation or solid insulation, and Basic air or solid insulation between all parts of the circuit and ground 2 Preventing access to all parts of the circuit by means of a conductive barrier, where the insulation between the floating/isolated circuit and the barrier is functional/operational insulation The conductive barrier need not be grounded Other schemes are possible, but these are probably the least-cost schemes Back to The Product Safety Newsletter If you would like to contact the IEEE Webmaster, to webmaster@ieeeorg Copyright 2001, IEEE Terms & Conditions Privacy & Security (DanRoman@ieeeorg) URL: (Modified:07/29/2003 ) M:/ /Floating Circuits - Protection ag 6/6