Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 114)

required with respect to the extent of risk to the protected object. This means playing safe and
“over protecting” the protected object with the result that its actual load capacity cannot be used
in full. In most cases this is anyhow not necessary.
An example is motor start-ups. They are usually so short that normal protective relays of class
10 or 10 A (
Tab. 4.1-3 ) can be used, although motors in most cases allow for longer starting
times without problems.
Thermal memory of electronic protective relays
Thermal protective relays, such as for example bimetal relays, have a thermal memory that
approximately simulates the cooling of motor windings. As bimetal relays do not require any
voltage supply, the thermal memory is assured even in the event of a voltage outage. This
means that for example after a short voltage outage (with the associated motor shutdown), the
bimetal strips are still warm and the next time that the motor starts it is protected against thermal
overload.
Electronic protective relays require a power supply to function. This can be obtained via current
transformers from the measuring circuit or via a power supply unit that is for example connected
to the control voltage supply. Electronic protective relays can be designed so that they have a
thermal memory or that they immediately are reset to “cold” when the motor to be protected is
switched off. Relays without thermal memory must be declared as such by the manufacturer
(marking on the device). For relays with a thermal memory IEC 60947-4-1 requires a test and
stipulates in this regard the following minimum requirements:
• Preheating
with I
e
• Disconnection of the current during 2 · T
p
(double time of trip class; for example 20 s for
class 10)
• Loading with 7.2 · I
e
• The relays must trip within 50 % of T
p
(for example 5 s for class 10)
Motor protection with heavy-duty starting
While the starting current of a motor (I
A
≈ 4 ... 8 I
n
) is determined by its design, the starting time
t
A
depends on the load (inertial mass and resistive torque). In accordance with Fig. 4.1-6
reference is made to heavy-duty starting, if the starting time – depending on the starting current
– is several seconds. Normal thermal motor protection relays are in such cases usually over-
loaded and will trip during start-up.
S
N
I
A
Fig. 4.1-6
Heavy-duty starts are classified as starting times above the limit curve (typical values)
t = starting
time
I
A
= starting current ( ≈ 4 ... 8 I
n
)
N = normal starting conditions
S = heavy-duty starting
LVSAM-WP001A-EN-P - April 2009 4-6