As shown in this app note:
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf
the suppressor (clamp) can be connected across the switch *or* across the
relay coil (inductor).
If the clamp is across the inductor, then a simple zener can't be used,
because the zener will conduct in the forward direction when the switch is
on; another diode must be placed in series with the zener. A bidirectional
suppressor (two zeners in one package, or a MOV) solves that problem, but
may be more expensive or hard to get.
If the clamp is across the switch, then a simple zener can be used, but as
shown in the paper I posted on ABSE, the dissipation in the clamp is not
just due to the energy stored in the inductance.
The thing to realize is that when the clamp is across the switch and the
switch turns off, the current in the inductor is decaying and the power
source, inductor and clamp are all carrying the same current. The
direction of current in the DC power source is such as to represent energy
flow *out* of the source. Since we believe in conservation of energy, we
see that the energy that must be absorbed by the dissipative elements
(mainly the resistance of the wire of the inductor and the clamp) will be
*not only* the energy stored in the inductor at turn-off. The energy
supplied by the power source during time while the inductor current is
decaying must be absorbed, as well as the inductor's stored energy.
The authors of the paper I posted over on ABSE show that if the clamp
voltage (when the clamp is across the switch) is only slightly higher than
the power source voltage, the time for the inductor current to decay is
increased, and the energy the clamp must absorb can be substantially more
than L*I^2/2.
The situation is different if the clamp is across the inductor, and this
factor should be taken into account when deciding where to place the clamp.
The increase in energy to be absorbed when the clamp, or snubber, is across
the switch is a phenomenon that also should be taken into account in
switching power supplies.