7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices
Chapter 7 Page 22 of 31
Section 4: Supervision and Monitoring Functions
4.1 Breaking Capacity Limit (50BCL)
Motors are generally controlled by three methods:
Larger rated motors typically at 3.3kV or 11kV use circuit breakers,
Medium to smaller rated motors use MCCB’s or fused contactors.
MCCB’s and Contactors both have limited fault breaking capacity. Contactors are typically limited to 8 to 10 times
their nominal current rating.
To prevent Contactor operation where fault current is above the contactor rating a check must be made that r.m.s.
currents are within the contactor rating, for currents above this level circuit breaking duty should be transferred to
the up-stream circuit Breaker. Alternatively, depending on circuit arrangement contactor operation can be time
delayed to allow fuses to clear fault current first then the contactor is opened.
If current is above this value then the Trip action is initiated from the 50 BCL output:-
50BCL is a high speed element, it’s instantaneous operation can be used to interrupt protections assigned as a
general trip (OUTPUT CONFIG > OUTPUT MATRIX > General Trip). All contacts assigned as ‘Gn **** Trips’ in
the OUTPUT CONFIG > TRIP CONFIG menu (thermal, P/F, E/F, Misc, Voltage, Freq, Power) are General Trips.
For example:
The 50-1 protection must not trip the contactor for currents in excess of say 6 xIn:
50-1 Element : Current setting and time delay as required. To recognise 50-1 as a ‘General Trip’ set as
‘OUTPUT CONFIG > TRIP CONFIG = P/F Trip’. Trip output to be connected to output relay assigned to
‘OUTPUT CONFIG > OUTPUT MATRIX > General Trip’.
50BCL Element: 6 x In. Output relay to be connected to suitably rated current break device.
4.2 Backspin Protection (81B)
The rotor of a pump motor may spin backwards when the pump is stopped. Starting the motor during this period
of reverse rotation (back-spinning) may result in motor damage. Backspin detection ensures that the motor can
only be started when the motor has stopped or slowed to within acceptable limits.
When backspin protection is used the Gn 81B Delay is always enabled. This delay setting must be coordinated
with the undervoltage or tachometer input when these are used.
4.3 Current Transformer Supervision (60CTS)
When a CT fails, the current level in the failed phase reduces to zero and the protection become unbalanced. The
CT output includes a large NPS component, however, this condition would also occur for a system fault. To
differentiate between the two conditions NPS voltage is used to restrain the CTS algorithm where the relay has
VT inputs.
A 3-phase CT failure is considered so unlikely (these being independent units) that this condition is not tested for.
Operation is subject to a time delay to prevent operation for transitory effects.
Where specific requirements are not available the default settings should be used:
Gn 60CTS Inps = 0.1 x In
Gn 60CTS Vnps = 10V
Gn 60CTS-I Setting = 0.05 x In
Summary of Contents for 7SR105 Rho
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