GE P642, Technical Manual

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4.17 RTD CONNECTIONS
Resistance Temperature Detector (RTD) inputs use screw clamp connectors. The connection block is situated at the
rear of the IED. It can accept wire sizes from 0.1 mm
2
to 1.5 mm
2
. The connections between the IED and the RTDs
must be made using a screened 3-core cable with a total resistance less than 10 Ω. The cable should have a
minimum voltage rating of 300 V RMS.
A 3-core cable should be used even for 2-wire RTD applications, as it allows for the cable’s resistance to be
removed from the overall resistance measurement. In such cases the third wire is connected to the second wire at
the point where the cable is joined to the RTD.
The screen of each cable must only be earthed (grounded) at one end, preferably at the IED end and must be
continuous. Multiple earthing (grounding) of the screen can cause circulating current to flow along the screen. This
induces noise and is also unsafe.
You should minimize the noise pick-up in the RTD cables by keeping them close to earthed (grounded) metal
casings and avoid areas of high electromagnetic and radio interference. The RTD cables should not be run
adjacent to or in the same conduit as other high voltage or current cables.
A typical cable specification would be:
● Each core: 7/0.2 mm copper conductors heat resistant PVC insulated
●
Nominal conductor area: 0.22 mm2 per core
● Screen: Nickel-plated copper wire braid heat resistant PVC sheathed
The following extract may be useful in defining cable recommendations for the RTDs:
Noise pick up by cables can be categorized into three types:
●
Resistive
● Capacitive
●
Inductive
Resistive coupling requires an electrical connection to the noise source. Assuming the wire and cable insulation are
in good condition and the junctions are clean, this can be dismissed. Capacitive coupling requires sufficient
capacitance to the noise source. This is a function of the dielectric strength between the signal cable on the noise
source and the power of the noise source. Inductive coupling occurs when the signal cable is adjacent to a wire
carrying the noise or it is exposed to a radiated EMF.
Standard screened cable is normally used to protect against capacitively-coupled noise. However for this to be
effective, the screen should only be bonded to the system ground at one point. Otherwise a current could flow and
the noise would be coupled into the signal wires of the cable. There are different types of screening available, but
the most commonly used are aluminium foil wrap, or tin-copper braid. Foil screens are good for low to medium
frequencies and braid is good for high frequencies. High-fidelity screen cables provide both types.
Protection against inductive coupling requires careful cable routing and magnetic shielding. The latter can be
achieved with steel-armoured cable and steel cable trays. The cable armour must be grounded at both ends so
the EMF of the induced current cancels the field of the noise source and shields the cables conductors from it.
However, the system ground must be designed such that it does not bridge two isolated ground systems. This
could be hazardous and defeat the objectives of the original grounding design. The cable should be laid in the
cable trays as close as possible to the metal of the tray. Under no circumstance should any power cable be in or
near to the tray. Power cables should only cross the signal cables at 90 degrees and never be adjacent to them.
Both the capacitive and inductive screens must be contiguous from the RTD probes to the IED terminals. The best
types of cable are those provided by the RTD manufacturers. These are usually three conductors, known as a triad,
which are screened with foil. Such triad cables are available in armoured forms as well as multi-triad armoured
forms.
Chapter 18 - Installation P64x
402 P64x-TM-EN-1.3