Siemens 7SK82, 7SK85 and 7SK80 Motor Protection Relays

Short answer

The Siemens SIPROTEC 7SK82 and 7SK85 are the motor protection relays of the SIPROTEC 5 platform: the 7SK82 for small and medium motors and the modular 7SK85 for motors of any size, with motor differential. The 7SK80 and 7SK81 are the motor relays of the SIPROTEC Compact range, and in older installations motors are also protected with the SIPROTEC 4 7SJ62.

What it is and where it is used

The Siemens SIPROTEC 7SK82 and 7SK85 are the motor protection relays of the SIPROTEC 5 platform. The 7SK80 and 7SK81 are the motor relays of the compact SIPROTEC Compact range. They are used on medium-voltage motors in industrial plants (pumps, compressors, fans, mills, conveyors), in pumping stations and in power plant station services. In older installations, many large motors are protected with the SIPROTEC 4 7SJ62, a multifunction relay that Siemens lists with motor functions such as restart inhibit and locked rotor.

For the background, see motor protection relay and induction motor and its protection.

Variants: 7SK82, 7SK85, 7SK80, 7SK81 and 7SJ62

Model	Platform	Application and main difference
7SK82	SIPROTEC 5	Small and medium motors (Siemens states 100 kW to 2 MW); fixed 1/3 base module, 4 current and 4 voltage inputs; up to two communication modules
7SK85	SIPROTEC 5	Motors of any size; modular, from 1/3 to 2/1 of 19-inch width; motor differential; up to four communication modules
7SK80 / 7SK81	SIPROTEC Compact	Medium-voltage motors; they are covered on the 7SJ80 page
7SJ62	SIPROTEC 4	Multifunction relay with motor functions; earlier generation

The 7SK82 and 7SK85 have ATEX-certified variants for motors in explosive atmospheres.

Main functions

Function	ANSI	What it provides on the motor
Stator and rotor thermal overload	49	Heating from load and starts (guide)
Starting time supervision and restart inhibit	48, 66	Long starts or starts too close together
Load jam	51LR	Load jammed while running
Unbalanced load	46	Heating from unbalance (guide)
Overcurrent and ground fault	50/51, 50N/51N	Short circuits and ground faults (guide)
Motor differential (7SK85)	87M	Internal fault in large motors (guide)

Siemens also states that the 7SK85 fault recorder stores up to 80 seconds at 8 kHz, useful for analyzing complete starts. Which functions each device includes depends on the configuration and the order code.

Communications and integration

The 7SK82 and 7SK85 use plug-in SIPROTEC 5 communication modules with IEC 61850, PRP and HSR redundancy and, depending on the module, DNP3, IEC 60870-5-104, Modbus TCP and IEC 60870-5-103. They offer phasor measurement (PMU) and role-based access control. They are configured with DIGSI 5. The 7SK80 and the 7SJ62 are configured with DIGSI 4. See Siemens DIGSI software and IEC 61850 GOOSE.

Frequently asked technical questions

7SK80 relay manual (PDF): the 7SK80 has a SIPROTEC Compact manual; the 7SK85 manual does not apply to it.
Thermal trip on starting: the motor data (starting current, permissible locked-rotor time, thermal constants) must come from the nameplate and the motor manufacturer’s datasheet.
The motor does not restart after a trip: the restart inhibit waits for the thermal model to cool down; it is a protection, not a relay fault.
Motor with a drive: metering and some functions change when the motor starts through a drive; check it in the configuration.
Migrating from 7SJ62 to 7SK85: the platform and the software change (DIGSI 4 to DIGSI 5); the settings and CFC logic are translated and tested.

The 7SK80 according to its manual (SIPROTEC 4, V4.78)

The manual of the SIPROTEC 4 Motor Protection 7SK80 (firmware V4.78, edition 11.2019, E50417-G1140-C344-A9, in English) describes a device that is often confused with the 7SK82 even though it is from another platform:

Scope. Protection and supervision of asynchronous machines of any size. It also serves on busbar feeders or as line protection on solidly grounded, low-resistance grounded, isolated or compensated networks. It covers radial lines with a single source, open or closed rings, and lines with sources at both ends. It is also backup for differential schemes of lines, transformers, generators, motors and busbars.
Basic motor function. It includes starting time supervision, restart inhibit (66) and load step function. It includes non-directional overcurrent with three definite-time stages and one inverse-time stage for phases and ground. It includes negative sequence, overload, breaker failure and ground fault. Depending on the variant ordered, frequency, overvoltage and undervoltage, and high-resistance ground fault are added.
Flexible functions. A set of user-definable protection functions; the manual uses one of them as an example of a reverse power application.
Motor statistics. It records operating and starting data.
Temperature. One or two temperature sensor boxes (RTD box) are connected through Ethernet on port A or through RS485 on port B; the connection setting must match that of the box.
Ports. Port A is used for DIGSI, NTP time synchronization and up to two SICAM 7XV5673 I/O units. Port B communicates with the control center through IEC 60870-5-103, PROFIBUS DP, DNP3.0 or Modbus; with the EN100 module, IEC 61850, PROFINET and DNP3 TCP are added.

If the nameplate says 7SK80, configuration is done with DIGSI 4 and the manual is this one, not that of the 7SK82 or 7SK85, which are SIPROTEC 5 and are configured with DIGSI 5. On a 7SK85, the record memory and the motor differential change what can be analyzed after a failed start. On a 7SK80, the available functions depend on the variant ordered. For that reason the order code (MLFB) is checked before assuming that a function exists.

If your equipment is discontinued

If your motor is protected with a 7SJ62 or another SIPROTEC 4, Siemens’ current motor line is SIPROTEC 5 (7SK82 and 7SK85) together with the compact range. Migrating requires translating settings and logic, reviewing inputs and outputs and the integration, and testing with the motor. We design the replacement and test it before start-up; see engineering design and studies.

How we can help

At Tiza Group we remotely configure and review SIPROTEC motor relays, worldwide, from the motor data and the approved settings, with a backup before and after the change. See remote configuration of Siemens SIPROTEC relays. If a motor tripped and the reason is unclear, send us the record for a trip analysis, or write to us through the contact page.

References

IEEE C37.2, device function numbers for power systems.
IEEE C37.96, guide for AC motor protection.
Siemens, product pages and profiles of the SIPROTEC 7SK82 and 7SK85; SIPROTEC 4 catalog (7SJ62) and SIPROTEC Compact catalog.

Frequently asked questions

What is the difference between the 7SK82 and the 7SK85?

Both are SIPROTEC 5 motor relays. Siemens presents the 7SK82 as a cost-optimized device for small and medium motors, with a fixed base module of four current and four voltage inputs and no expansion. The 7SK85 is modular, for motors of any size, adds motor differential and supports more input, output and communication modules.

Where can I find the 7SK80 manual in PDF?

The 7SK80 belongs to SIPROTEC Compact and is configured with DIGSI 4; Siemens publishes its manual by firmware version. The 7SK82 and 7SK85 are SIPROTEC 5, have a different manual and are configured with DIGSI 5.

Is the 7SK85 suitable for motors in hazardous areas?

Siemens offers ATEX-certified variants of the 7SK82 and 7SK85, intended for motors in explosive atmospheres. The certification applies to the variant ordered, not to any device of the family.