Construction of Giresun Wastewater Treatment Plant  Tender Dossier Volume 3 Employer’s Requirements  Section 08: Electrical Works  39 of 124  8.8.1 General  8.8.2 Signalling and Protection Circuits  8.8.3 Instrumentation  8.8.4 Oxygen measurement system  In the following are listed the minimum requirements for the instrumentation. Instruments shall be provided  together with original installation accessories.  Every digital signal in plant shall be provided as a volt free contact wired to terminal and Ethernet. A fault and  alarm situation shall be signalled via opening of a contact. The opposite i.e. by closing a contact will not be  accepted. Operation is to be signalled by closing of a contact.  All faults, alarms and operation signals shall be wired to the PLC.  All 4-20 mA analogue signals from transmitters, measuring units, sensors etc. shall be wired to the PLC through  loop isolators to assure galvanic separation.  Galvanic isolators will be used in all analog inputs and outputs. PLC output slim relay will be used at PLC  digital outputs.  Important : Every protection circuit shall be hardwired and based on the quiescent current principle i.e. when a  fault is detected the control current is interrupted resulting in the disconnection of the equipment to be  protected.  Instruments shall have an analogue output signal 4-20 mA and a volt free error /fault contact to open in the  event of malfunction. Accuracy shall for all instruments be better than 0.5 %. LCD indicators shall be provided  together with instruments for reading in the field. At least IP65 degree shall be provided. Oxygen transmitters shall be provided for measuring of the oxygen content and measuring range shall  be 0-20 mg/l-ppm.  Measuring principle shall be optical. Transmitter shall include robust plastic housing. It shall be possible for  functionality modular extendable at any time. Transmitter shall be IP 66/67 for the field conditions and its  operating condition shall be suitable for modular 4 wire controller. Transmitter ambient temperature shall be  between -20 $^{0}$C and 60 $^{0}$C. Common communication protocols and interfaces, such as 0/4...20 mA and Ethernet  or 0/4….20mA/HART shall be integrated. It shall be possible for all non-ex applications.  The analogue measuring of dissolved oxygen shall be based on a sensor with a multi electrode system.  8_Vol3_d4 3_Section 8 electrical works_en_GIR rev6.doc  Values are capable of reading from transmitter display at eye level ( above 1.6 meter from floor level).  All instruments supply voltage shall be 24 V DC.  All supply and signal cable shall be the same cable. This cable shall be screened type. (LIYCY)  The level switches shall be the floating type with a built-in change-over contact system.  The contact system shall be casted in a polypropylene housing in protection class IP67.  TCP/IP or analog signals can be used for signalling to PLC.  Instruments to be used in Exproof areas will be exproof type.  High level alarm for the reservoir shall be with a level switch. 