The DOLD SP 9278.12 Series Current Asymmetry Relay (German: Strom-Asymmetrierelais / English: Current Unbalance Relay) is an engineered industrial protective instrument researched and manufactured by E. DOLD & SÖHNE GmbH & Co. KG in Germany. Within industrial electrical plants, motor breakdowns and electrical heating element failures often stem not from incoming line voltage deviations, but rather from severe current unbalance among phase conductors. Such current asymmetry frequently arises from turn-to-turn winding insulation degradation, unbalanced cable impedances, or unequal mechanical phase loading. When phase currents diverge even slightly, internal thermal dissipation within motor stator cores surges exponentially, rapidly accelerating insulation embrittlement, inducing rotor mechanical vibrations, and triggering catastrophic motor burnout that conventional voltage relays fail to detect.

The DOLD SP 9278.12 series is engineered to eliminate these hazards through continuous direct current measurement across all three phases within an operational range of AC 1A to 15A. The device performs real-time percentage comparison between phase currents. Whenever current unbalance exceeds the calibrated front setpoint, the precision electronics initiate an adjustable time delay Tv (infinitely settable from 0.1 to 20 seconds via a front potentiometer). This built-in switching delay filters out transient current surges during heavy motor startup sequences and transient load spikes, preventing nuisance tripping and unnecessary production downtime.

Constructed in an industrial S-model enclosure with a 70 mm width, the SP 9278 features terminals located at the top of the housing, optimized for mounting plates and cable duct runs in control panels. Its ".12" contact architecture provides two independent changeover contacts (2 C/O) with high switching capacity, enabling concurrent disconnection of the motor contactor coil and transmission of telemetry alarm signals to central SCADA/PLC systems. This entry covers three distinct variants:
- DOLD SP9278.12 AC/DC 24V AC 1-15A (Tv=0.1-20s): Powered by an AC/DC 24V auxiliary supply, ideal for low-voltage PLC systems and SELV/PELV safety circuits.
- DOLD SP9278.12 AC 220-240V AC 1-15A (Tv=0.1-20s): Operates directly from standard AC 220-240V mains supplies without requiring external power transformers.
- DOLD SP9278.12/010 AC 220-240V AC 1-15A (Tv=0.1-20s): Enhanced variant equipped with an integrated fault latching memory function (Fehlerspeicher), which maintains output contact tripping after a current unbalance event until an authorized technician performs an intentional manual reset.

Housed in a flame-retardant industrial thermoplastic enclosure for 35mm DIN-rail or screw mounting, and equipped with clear front diagnostic LEDs for supply and current trip status, the unit complies with IEC/EN 60255-1. The DOLD SP 9278.12 provides reliable protection for heavy-duty pumping stations, compressors, industrial resistance furnaces, and critical drive systems worldwide.

• Manufacturer: DOLD (E. DOLD & SÖHNE GmbH & Co. KG)
• Country of Origin: Germany
• Product Series: VARIMETER SP 9278
• Technical Classification: 3-Phase Current Asymmetry Relay (Strom-Asymmetrierelais)
• Contact Code: .12 (2 Changeover contacts - 2 C/O)
• Models Covered in this Entry:
  - DOLD SP9278.12 AC/DC 24V AC 1-15A Tv=0.1-20s
  - DOLD SP9278.12 AC 220-240V AC 1-15A Tv=0.1-20s
  - DOLD SP9278.12/010 AC 220-240V AC 1-15A Tv=0.1-20s (Variant with fault latching memory)
• Monitored Power System: 3-Phase AC networks
• Measuring Current Range: AC 1 ... 15 A
• Nominal Measuring Frequency: 50 / 60 Hz
• Auxiliary Supply Voltage (UH):
  - AC/DC 24V model: Tolerance range 0.85 ... 1.15 UN
  - AC 220-240V models: Tolerance range 0.85 ... 1.15 UN, 50/60 Hz
• Monitoring Functions:
  - Current asymmetry / unbalance across 3 phases
  - Current load interruption and phase unbalance detection
  - Variant /010: Integrated latching fault memory (manual reset required)
• Asymmetry Setting Range: Infinitely adjustable via front-panel potentiometer
• Switching Delay (Tv): Adjustable response time from 0.1 to 20 seconds
• Contact Switching Capacity: AC-15: 3A / AC 230V; DC-13: 2A / DC 24V (compliant with IEC/EN 60947-5-1)
• Thermal Continuous Current (Ith): Max. 5A
• Enclosure Architecture: S-model housing, 100 mm depth with terminals positioned at the top
• Module Dimensions (W x H x D): 70 x 90 x 100 mm
• Mounting Profile: Snap-on mounting for standard 35 mm DIN-rail (IEC/EN 60715) or screw mounting
• Terminals: Vibration-proof box clamp screw terminals designed for secure industrial cabling
• Protection Degree: Housing IP40, Terminals IP20 (according to IEC/EN 60529)
• Ambient Operating Temperature: -20°C to +60°C
• Compliant Safety Standards: IEC/EN 60255-1, IEC/EN 60947-5-1, CE
• Common Industrial Deployments: Heavy 3-phase motors, air compressors, industrial heating elements, submersible pumps, and power substations