| TECHNICALLY
SPEAKING
by Richard Nute
© 1998 Richard Nute
FLOATING CIRCUITS
– PROTECTION AGAINST ELECTRIC SHOCK
Introduction
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, Sub-clause 1.2.8.3) and hazardous current
(exceeds the limits for Limited Current as defined in IEC
950, Sub-clauses 1.2.8.6 and 2.4.2). 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 1.2.8.6 and
2.4.2.
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.)
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.
If we interpose Basic Insulation
between one pole of the circuit and the man, then we
prevent shock current through the body. See Figure 3.
Notice that the opposite pole is still
accessible. But, the Basic Insulation prevents shock
current through the body.
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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 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.
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 Insulation (because it
provides insulation against the SECOND body connection).
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 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-to-opposite pole and
pole-to-ground-to-opposite 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.
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.
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!
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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!
Conclusion.
We have shown two simple,
cost-effective schemes for protection against electric
shock from a floating or isolated circuit:
- 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.
- 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.
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