Current detective

Introduction – Current Detective

The Current Detective project is a method to monitor the small leakage current that would cause Ground Fault Circuit Interrupt to trip.

Ground Fault Circuit Interrupt (GFCI) or Residual Current Device (RCD)

What is a GFCI or RCD?

It is an electro mechanical device that monitors current flow on the hot and neutral legs of a circuit. When current between the two legs is greater than 6mA, the device causes the circuit to “trip”, or creates an open or break in the circuit, effectively stopping current flow.

What Does a GFCI Monitor?

A GFCI monitors the difference between the hot lead current and the neutral lead current in an outlet. If there is a difference of 5 to 6 milliamps, it will “trip” the outlet so the electricity path is opened. A GFCI will also monitor the current on the neutral lead, and if that current is not present on the hot lead, this will cause an immediate “trip”.

How Much Current Hurts???

(NEMA)

• 0.5–2mA
  • Threshold of perception
• 2–10mA
  • Painful sensation, increasing with current.
  • Muscular contraction may occur, leading to being thrown-off
• 10–25mA
  • Threshold of ‘let go’, meaning that gripped electrodes cannot be released once the current is flowing.
  • Cramp-like muscular contractions.
  • May be difficulty in breathing leading to danger of asphyxiation from respiratory muscular contraction
• 25–80mA
  • Severe muscular contraction, sometimes severe enough to cause bone dislocation and fracture.
  • Increased likelihood of respiratory failure.
  • Increased blood pressure.
  • Increasing likelihood of ventricular fibrillation (uncoordinated contractions of the heart muscles so that it ceases to pump effectively).
  • Possible cardiac arrest
• Over 80mA
  • Burns at point of contact and in internal tissues.
  • Death from ventricular fibrillation, cardiac arrest, or other consequential injuries

Please note: Most GFCI’s on the retail shelf do not have over current protection. If this is required, look for one with a circuit breaker or built in fuse.

Transformer Basics

A transformer operates on the principles of “electromagnetic induction”, in the form of mutual induction. Mutual induction is the process by which a coil of wire magnetically induces a voltage into another coil located in close proximity to it. A transformer consists of into of two sections of coiled wire that are separated from each other, a primary winding and a secondary winding. As the AC current on the primary winding goes through the 360º, 60 Hertz cycle, it will cause a current to be generated in the secondary winding. The ratio of primary to secondary windings or “turns” will determine the amount of current passed from the primary to the secondary. Each turn is one loop of the coil.

Current Sense Transformer

A current sense transformer will pass one “leg” through the coil and the second leg around the coil. The primary winding of the transformer is the single wire. The current sense coil is typically a 2,000 or more “turn” coil. This allows for capture of the primary current in the secondary without impacting the primary.

GFCI Current Sense

A GFCI will pass both legs through the sensing coil. By doing this, the difference between the hot and neutral legs is reflected as the hot and neutral legs are 180º out of phase, effectively canceling each other out.

Basic Residential Electricity

Electricity needs a round-trip path from the source, or the current doesn’t flow. In a house there are 3 wires.

• Hot
• Neutral
• Ground

If you have looked in a residential electrical panel, you may have noticed that the neutral and ground are tied to the same point electrically.

GFCI Overview

GFCI – Normal Operation

GCFI Test Button

GFCI Something Went Wrong

GFCI Summary

• GFCI outlets save lives by monitoring the hot and neutral legs of an outlet to ensure that the current matches within 6mA.
• GFCI outlets will open the circuit in 25mS to prevent electrocution.
• GFCIs will detect ground to neutral connection downstream from the outlet.
• GFCIs will not protect against overload on the hot/neutral unless specifically built to do so.

Current Detective – Concept

• The Current Detective is a circuit that is inserted in line between the GFCI outlet and the load.
• It monitors the hot and neutral legs by passing both the hot and neutral through a current sensing coil.
• If any difference in the current levels is sensed, it is converted to voltage on a resistor and amplified.
• The goal is to make something simple to build and operate.

This design uses a current sense coil with both the hot and neutral leads passing thought it. The ground wire is run outside of this coil. A TLC082 operational amplifier actually provides 2 high gain amplifiers in one unit. One amplifier set for a 100 gain, and the second is adjustable to allow for calibration. The meter in this schematic is a digital voltmeter set to 20volts. The power supply portion of the circuit is 2 9volt batteries to provide portability. This design has no direct connection to the high voltage side. The ground wire could be skipped but accommodations would need to be made to support leakage current to ground for calibration.

Current Detective Schematic – Normal Operation

In normal operation, the current on the hot lead and the neutral legs will be equal. The sense coil output will be zero since the 2 different phases are 180º out of phase and will cancel out each other. The meter should read 0 volts when calibrated.

Current Detective Schematic – Leakage

In an operation with .01 amps of leakage; the sense coil will output the difference of the 2 phases of power. This current will be converted to voltage by the 1K across the coil. This voltage will be amplified 100 times by the first amplifier. The second amplifier is adjusted to have 1 volt reading equal to 1 milliamp of leakage. Keeping in mind that greater than 5mA of leakage is a GFCI “trip”.

Outlet Tester

The 3 prong outlet testers use neon bulbs between the hot, neutral and ground to validate wiring and indicate problems. These testers may allow 2 to 5 milliamps between the hot and ground. To a GFCI, this will appear as leakage.

Two Outlet Test Plugs

Two outlet test plugs will allow approximately 5milliamps from the hot to the ground wire. Again, the GFCI would consider this leakage current. This will be close to tripping the GFCI.

The Current Detective test plug

This circuit calls for a controlled calibration. A combination of a resistor and neon bulb with a known current draw will provide leakage current of a known quantity to allow the calibration. This will create a small controlled load from hot to ground (leakage).

Parts list

• C2A-ND (NE-2H) Neon bulb
• 1.9 milliamps current (designed)
• 30K ohm resistor

Reference material

https://www.nema.org/Products/Documents/NEMA-GFCI-2012-Field-Representative-Presentation.pdf