# Why shunt resistance has 3 line? I know that the there is two lines to connect it to the integrator like the below picture , so what the 3rd line do ?? how to use it ?? is it necessary ? • I can't find a datasheet for it ... It doesn't have any part number or anything .... just a plain metal ... – xsari3x Nov 12 '11 at 13:36
• I'm puzzled by three wires. 2 or 4 would make sense. 4 leads from a high current shunt are not unusual. That allows you to measure the voltage drop accross the resistance only without the drop accross any connections getting in the way. Maybe this is a special part and the third wire is a tap part way to provide alternal resistance? What do you get when you measure with a low value Ohm meter? – Olin Lathrop Nov 12 '11 at 13:55
• High sensitivity and low sensitivity measurements? You might find the resistance between (blue/yellow) and (blue/green) is different by a factor of 10. – Optimal Cynic Nov 12 '11 at 21:04
• The 3-wire configuration most certainly is a variant of a normal 4-terminal-sensing circuit, like here: en.wikipedia.org/wiki/Four-terminal_sensing (scroll down). – 0x6d64 Nov 14 '11 at 8:11
• @0x6d64, write that. It has two terminals that are higher power for your connection(the screw terminals you noted), then three terminals on the device for measurement. That is 5 terminals. – Kortuk Nov 14 '11 at 18:00

These are current shunt device sold for use with KWh meters. As Kortuk notes, these are 5 wire devices - not 3 as may appear.

2 of the 5 "wires" are hard bolted connections to the "copper" frame.

Current flows between studs or bolts which are inserted through the two holes so the main current flow is via the copper strip.

If you take the diagram you provided at face value (see anotated version below) it shows exactly what is being done.

• The two stud holes make contact with the power circuit.
B is the power input side. C is the load side
Th sense element is usually constantan.

• So BC measure a voltage proportional to load current.

• A is used to measure the input voltage = the common mode voltage in the diagram. The diagram does nit show A being used BUT in a "real" system the 2 x R2 resistor could be returned to A if desired instead of ground.

• Va and Bc are very similar - the voltage drop across half the terminal is the only difference, This could be used but may not be.

Note that A & C leads have the same colour code in all 3 devices while B varies.

B & C leads connect hard up against the sense element. The voltage drop that they see is caused essentially solely by the sense element. If you want to sense current based solely on this devices sense element these are the leads to use.

However, the A lead connects to the side of the bolted portion at one end. There will be minimal current flow in this area so it will assume the potential of the input "stud" less any contact voltage drop. There will be SOME voltage drop across the "copper" frame but this would usually be liable to be small. However, what voltage drop there is will appear across leads A-B and be able to be measured by the test system.

As per diagram, A is used for voltage sensing while B-C is for current sensing. kWh meter PCB & resistor  What it does seems clear enough.
The question is, still, why?

Possible only:

• If there is temperature rise in the copper then the ratio Vcopper / Velement will vary.
• The voltage Va-b will be small in all cases. This could allow monitoring in a different way to the higher voltage Vb-c.

The above sold here

Similar sold here

kWh meter PCB for use wih these devices

• I tip my hat for you – xsari3x Nov 17 '11 at 23:43
• Thanks - but @Kortuk 's comment about it being a 5 wire device (as is obvious from the diagram) is the main thing to realise. The rest is useful followon. – Russell McMahon Nov 18 '11 at 0:59
• I didn't get what u mean by the 5 wire device ? – xsari3x Nov 18 '11 at 1:38