Brochure

ESG
8.548.54
8.548.54
8.54
Surge ratings and high inrush
current loads
After improper heat sinking, surge current is one of the
more common causes of SSR failure. Overstress of this
type can also seriously impair the life of the SSR. There-
fore, in a new application it would be wise to carefully
examine the surge characteristics of the load.
There are very few completely surgeless SSR loads.
Resistive loads, such as heating elements and incande-
scent lamps, can prove problematic. Capacitive loads
can also prove equally problematic because of their
initial appearance as short circuits. High surge currents
can occur while charging, limited only by circuit resi-
stance. Inductive loads, on the other hand, tend to
impede high inrush currents, in fact, inductance is often
inserted into a circuit for the express purpose of limiting
high fast rising peak currents (e.g. EMI filters, chokes,
etc.). However, inductive loads can give rise to high
inrush currents.
Inductive loads have traditionally created more pro-
blems on turn-off rather than turn-on due to stored
energy and "back EMF". The inherent zero current turn-
off characteristics of thyristors used in ac SSRs is most
beneficial in this regard.
Surge ratings
The highest surge current rating (As) of an SSR is
typically 10 times the steady-state RMS value, and it is
usually given as the maximum nonrepetitive peak
current for one line cycle. It should be noted that a surge
of this matgnitude is allowable only 100 times during the
SSR lifetime. The preceding cautionary notes would
tend to reduce the attractiveness of the high surge
capability (100%) of the ac SSR; however, they apply
only to the extreme limits where the SSR should not be
designed to operate anyway. When a reasonable surge
safety margin is used, conditions rapidly improve.
Generally, dc SSRs do not have an overcurrent surge
capability, since the output transistors (non regenerative)
are usually rated for continuous operation at their
maximum capacity. The tendency is for the dc SSR to cut
off (current limit), thus impeding the flow of excessive
current. However, the resultant over-dissipation may
destroy the relay if the surge is prolonged. If overcurrent
carrying capacity is required, as may be the case when
designing fuse protection, the SSR could be over speci-
fied (have a higher current capability).
To aid in the proper design of SSR fusing, an I
2
t rating is
usually given. This parameter expressed in A
2
s is useful
since it can relate directly to the published fuse charac-
teristics.
It is generally derived from the peak surge (one cycle)
output thyristor ratings as follows:
where:
I = peak surge current – (sinusoidal)
t = duration of surge (normally 8,3 μs)
(0,0083 seconds in the formula)
For example, for a 25 amp SSR with a 250 amp one-
cycle surge rating, the value would be 260 A
2
s.
Inductive loads
High inrush lamp and capacitive loads sometimes
include a series inductor such as a choke or transformer.
This will tend to limit the initial inrush current, but the
combination will primarily be seen by the SSR as an
inductive load. While most SSR loads, even lamps,
include some inductance, its effect with resistive loads is
usually negligible. Only those loads that utilize magne-
tics to perform their function, such as transformers and
chokes, are likely to have any significant influence on
SSR operation.
The majority of SSRs will operate inductive loads with
power factors as low as cosϕ 0,3, especially if they are
switching medium to high current loads relative to their
rating. SIG Positec Systems relays are 100% tested for
operation at cosϕ 0,5. When a load is so light that its
rating is close to the minimum current rating of the SSR,
the off state leakage may become significant when
compared to the load current. The leakage may have a
deleterious effect on certain loads such as small soleno-
ids that fail to drop out, or motors that buzz or even
continue to run. The solution is to reduce the load
impedance by means of a shunt or parallel impedance,
thus reducing this voltage below the drop out or off
threshold of the load.
A saturating inductive load can also cause switching
problems with the SSR. The ac impedance of such a load
is relatively high under normal conditions. However,
when saturation occurs the inductance falls to a very low
value, resulting in a fall in impedance close to that of the
Copper resistance of the coil winding. This can cause
several cycles of surge currents in excess of 30 times the
steady-state value, which may serioulsy affect the
lifetime.
Transformer switching
Extremely high current surges are commonly associated
with transformers, especially those with a penchant for
saturation. The zero voltage turn-on feature of standard
SSRs can increase this possibility and might require that
special precautions be taken.
I
2
t =
I
2
PK
t
2