Analogue line testing
How to tell a dead line from dead equipment, and an inside-wiring fault from a carrier fault, using a butt set and a multimeter. Covers on-hook voltage, loop current, ring voltage and the polarity check, what each reading should be, and what a wrong one means — including the readings that say the pair is shorted, open, or crossed with another line.
Doing it
Four measurements, in the order that narrows the fault fastest — and the one that tells you whether to call the carrier.
- Butt set with monitor mode
- Multimeter reading AC and DC volts
- Toner and probe
- 01
Go on-hook first and measure across the pair
With nothing off-hook, measure DC volts across tip and ring. Expect roughly 48 V — anything from about 42 to 56 is normal. Near zero means no battery from the exchange or an open pair. Around 5 to 10 V usually means something is already off-hook somewhere on the line.
CautionRing voltage can exceed 90 V AC. Do not hold both conductors while a call is coming in.
- 02
Check polarity while you are there
Tip should read positive with respect to ring on a correctly wired line. Reversed polarity still passes a call, which is why it survives, but it upsets some fax machines, older modems, and coin and message-waiting circuits.
- 03
Go off-hook and read loop current
Put the set off-hook and measure current in series, or read the voltage drop. Off-hook voltage should collapse to roughly 5 to 10 V and loop current should land between about 23 and 60 mA. Under 20 mA and the line is too long or too resistive to hold a reliable call — that is a carrier or a wiring problem, not a phone problem.
- 04
Listen before you conclude
Monitor mode on the butt set is the fastest instrument you have. Hum on the pair points at a grounded conductor or an unbalanced pair. Crosstalk from another conversation means a split or a crossed pair at a block. Complete silence with normal on-hook voltage often means the fault is downstream of where you are standing.
CautionBridge in monitor mode, not talk, on a line you do not own. Going off-hook on a live circuit drops whatever call is on it.
- 05
Split the building in half
Disconnect at the demarcation point and test the carrier side alone. Normal readings there and a fault inside means the problem is yours; a fault at the demarc means it is theirs, and you now have the one measurement that gets a carrier ticket taken seriously.
- 06
Test the pair itself, not just the line
With the pair disconnected at both ends, check continuity end to end, resistance between the two conductors, and resistance from each conductor to ground. A short reads near zero between conductors; an open reads infinite along one; a wet or damaged pair shows a low reading to ground and is the classic intermittent that only fails when it rains.
- What should a phone line read on a multimeter?
- About 48 V DC across tip and ring on-hook, dropping to roughly 5 to 10 V off-hook, with 20 to 60 mA of loop current. Ringing is a superimposed AC voltage that can exceed 90 V at around 20 Hz. Anything far outside those bands points at a specific fault rather than a vague one.
- The line has voltage but no dial tone. What now?
- Voltage without dial tone usually means the pair reaches the exchange but the loop cannot close — a high-resistance joint, a corroded punchdown, or a partial open. Go off-hook and read the current: if the voltage does not collapse and current stays under about 20 mA, the loop is not closing and the fault is in the path, not the phone.
- Does reversed polarity matter?
- Not for a normal voice call, which is why it goes unnoticed for years. It does matter for some fax machines, older modems, message-waiting indicators, and anything expecting a specific tip and ring orientation. It costs nothing to correct while you are already at the block.
- Is it safe to test a live line?
- With sensible care, yes — the on-hook voltage is low. The hazard is ring voltage, which can exceed 90 V AC and arrives without warning. Do not bridge both conductors by hand, use a set with a proper monitor mode, and treat any line feeding an alarm or elevator as one you do not interrupt.
The rest of the section
- 01RJ45 Wiring: T568A and T568BBoth colour codes, pin by pin, side by side.
- 02RJ11 Wiring and Phone PinoutsSix positions, three lines, two colour systems.
- 03Keystone Jack TerminationStrip, seat, punch, snap — without failing the certification.
- 0466 Block WiringFifty rows, four columns, and where the bridging clips go.
- 05110 Block WiringLay the pairs in, press the connecting block on.
- 06Cat5e vs Cat6 vs Cat6a vs Cat8Speed, distance, and power decide it — not the bigger number.
- 07Fibre Types, Connectors and PolishRead the jacket, read the connector, don't mate blue to green.
- 08Subnetting, ExplainedWhere the numbers come from, and the meaning behind them.
- 09PoE Standards and Why It Won't PowerThe switch reserves the class, not the draw.
- 10Camera Lens, Field of View and Pixel DensityCovering the room is not the same as identifying a face in it.
- 11Patch Panels and Labelling to TIA-606A label that survives is worth more than a tidy rack.
- 12Rack Planning: Units, Depth, Weight and HeatIt fits vertically. That was never the hard part.
- 13Writing a Switch ConfigurationFrom factory default to a switch you would hand over.
- 14Networking and Cabling GlossaryThe terms, and what they mean on a job.
- 15POTS to VoIP: What BreaksThe lines nobody remembers until they stop working.
- 16Toning and Tracing a CableFinding one pair in a bundle of two hundred.
- 17Reading a Cable Certifier ReportWhat PASS* actually means, and which number to look at first.
- 18Which Cable Tester Do You Need?Three tiers, and the one most people actually need.
- 19Running an Office Floor RestackEveryone sits down Monday and their machine works.
- 20Cable Management for Desks and RacksTidy is a by-product. Serviceable is the point.
- 21Asset Tagging and CMDB AuditsThe scan is easy. Scanning it into the right record is the job.
- 22Cabling an Office Fit-OutHow many drops, run where, and what it takes to add one later.
- 23Parcel Locker Site SurveysWhether the unit fits, powers, connects — and gets in the door.
- 24VLANs and TrunkingAccess, trunk, tagged, native — and which one is the security hole.
- 25Spanning Tree and Switching LoopsWhy one patch cord can take a whole floor down in seconds.
- 26Reading a TracerouteWhich loss is real, and whose network it is in.
- 27DHCP, DNS and Helper AddressesWhy the new VLAN gets no addresses, and why it is always DNS.
- 28Wi-Fi Channel Planning and AP PlacementMore access points at lower power, not fewer at maximum.
- 29SMART Attributes That Predict FailureA drive can report PASSED while it is visibly dying.
- 30BitLocker: Keys, Recovery and EscrowThe key exists somewhere, or the data does not.
- 31Grounding and Bonding a Telecom RoomOne ground, one path, and no loops between rooms.
- 32UPS Sizing: VA, Watts and RuntimeThe number on the box is not the number you need.
- 33Mounting Displays on Any WallFind the structure. Everything else is a repair bill.
- 34Mounting IP CamerasHeight, angle, and the surfaces that ruin the picture at night.
We do this work
as well as document it
Anyone standing at a block with a dead extension, trying to establish whether to call the carrier or keep walking the building.