> For the complete documentation index, see [llms.txt](https://help.security.gallagher.com/knowledge-hub/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.security.gallagher.com/knowledge-hub/deployment-resources/install-and-commission/wiring-basics/wiring-primer.md). # Wiring Primer [Compliance & Reference](broken://pages/xT7N7IT3BP4gxJvnsCrN) › [Wiring Basics](/knowledge-hub/deployment-resources/install-and-commission/wiring-basics.md) › Wiring Primer ## This page answers * What do voltage, current and resistance mean? * Why is a Gallagher input wired with two resistors? * Why is the default resistor value 4k7? * How much resistance does the cable itself add? A basic understanding of how electricity flows through wires and around circuits explains why inputs and outputs have to be wired a certain way. This page is for readers wiring Gallagher hardware without an electrical background. ## Voltage, current and resistance Electron flow is not something you can see, so use water as the analogy. * A **tank of water** is the supply, like a battery. * A **hose** is the wire that conducts the electricity. * A **tap** is a switch that controls the flow. * The pressure at the tap is **Voltage**, measured in Volts. * The flow rate through the hose is **Current**, measured in Amps. * The effect the hose diameter has on that flow is **Resistance**, measured in Ohms. The three are related by one formula: ``` V = I × R (Volts = Amps × Ohms) ``` Squeeze the hose to increase the resistance and the flow rate drops. The same happens with electricity. ## Completing the path In an electric circuit the electrons flow from the negative to the positive terminal of the supply. They take the path of least resistance. ![Primer example circuit — positive and negative rails with R1 in series and R2 in parallel with switch S1, i.e. a dual end-of-line input configuration](https://3862776825-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Feem9HLOjc53n2VxEz3Cq%2Fuploads%2Fgit-blob-76c76c3b6859197f6770caca06618f993a12f4b7%2FSECHUB-CR-WIRING-PRIMER-01-dual-eol-example.jpg?alt=media) The circuit above has two resistors of 1Ω each and a switch, S1. With a supply of 1V: * **S1 open.** Resistors in series are added together, so 1Ω + 1Ω = 2Ω. Current = V/R = 1V / 2Ω = **0.5A**. * **S1 closed.** The electrons take the switch instead of R2, leaving 1Ω. Current = 1V / 1Ω = **1A**. ## What does all this mean? That circuit is a **dual end-of-line** configuration for a Gallagher input, so called because there are dual resistors. It is how the I/O device detects the different states of an input point: it constantly measures the current the input point is drawing. The default resistors used in Gallagher systems are **4,700Ω** resistors. The value is standard on the E12 resistor standard. Other systems use values from 1,000 to 10,000 Ohms. Writing 4,700 Ohms is repetitive, so values are written as **4k7**, **1k** or **10k**, where **k** means multiply by 1,000 and **M** means multiply by 1,000,000. Gallagher hardware inputs supply **5V**. A system using the default resistors therefore expects to see: | Condition | Resistance | Current drawn at 5V | | --------- | ---------------- | ------------------- | | Normal | 4,700Ω | 0.001A | | Alarm | 9,400Ω (**9k4**) | 0.0005A | ## Is there anything else I should consider? Copper wire is not a perfect conductor. The wire is itself a resistor, and the longer and thinner the wire, the more resistance it adds to the circuit. Remember that the run is to the detector **and back**. | Cable | Resistance | Added by a 100m run | | --------------- | --------------------- | ------------------- | | 24 AWG / 0.5mm² | about 0.08Ω per meter | 16Ω | | 0.22mm² | about 0.4Ω per meter | 80Ω | *** 🔗 **Related** * [Wiring Inputs](/knowledge-hub/deployment-resources/install-and-commission/wiring-basics/wiring-inputs.md). The resistor configurations this primer explains, and the states each one detects. * [Wiring Outputs](/knowledge-hub/deployment-resources/install-and-commission/wiring-basics/wiring-outputs.md). What a Gallagher relay output can switch. Next: [Wiring Inputs](/knowledge-hub/deployment-resources/install-and-commission/wiring-basics/wiring-inputs.md) → *Source: SECHUB-CR-WIRING-PRIMER* --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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