A link light on the switch proves very little. A run can light up and still drop packets, fail under PoE load or hide a wiring fault that shows up after the walls are closed. Knowing how to test an Ethernet cable properly means picking the right level of test for the job, reading the results correctly and handing the client something that proves the work.
How do you test an Ethernet cable?
Here is how to test an Ethernet cable: disconnect both ends from network equipment, connect a tester’s main unit and remote, and run a wiremap to find opens, shorts, reversed, crossed or split pairs. Use a qualification tester to confirm the run supports the target speed, or a certifier where the client or warranty needs category certification. Save and hand over the results.
Before you test: isolate the run
Unplug both ends of the cable from switches, routers, cameras and computers before connecting a tester, so the test sees only the cable and its terminations. For a permanent link, connect at the patch panel and the wall jack; for a patch cord, plug both ends straight into the tester.
Write down the cable identifier before you start, so every result can be tied back to a wall plate and a panel port. Our guide on how to label network cables covers the scheme.
Cable tester vs certifier: three levels of testing
Field testers fall into three tiers. Each one includes what the tier below it does, and each answers a different question.
| Level | Question it answers | What it checks | Typical use |
|---|---|---|---|
| Verification (wiremap) | Is it wired correctly? | Continuity, opens, shorts, pair order; some units add length and split pairs | Every termination, troubleshooting |
| Qualification | Will it carry this speed? | Wiremap and length, plus whether the run supports a named Ethernet speed | Upgrades, small jobs, proving a suspect run |
| Certification | Does it meet the category standard? | Wiremap, length, insertion loss, crosstalk, return loss and more against TIA limits | Commercial projects, manufacturer warranties, specs that require it |
A verification tester is enough to prove a termination. It cannot tell you whether a Cat6 run actually performs as Cat6; only a certifier measures that against the category limits. Our Cat6 vs Cat6A guide explains what each category is meant to support.
Network cable wiremap test: what each result means
A wiremap checks that each of the eight conductors connects to the matching pin at the far end, one to one through eight to eight. The common faults:
- Open: a conductor does not reach the far end. Usually a wire not seated in its slot, a nicked conductor or a broken pin.
- Short: two conductors touch. Often a stray strand or a crushed cable.
- Reversed pair: the two wires of one pair are swapped at one end, for example pin 1 to pin 2.
- Crossed pairs: a whole pair lands on another pair’s pins, typical of one end in T568A and the other in T568B.
- Split pair: pin to pin is correct, but the conductors are not paired as the standard requires.
Crossed pairs on a permanent link are almost always mixed schemes; T568A vs T568B shows both pinouts. Opens and shorts at a jack usually mean re-terminating it, as in how to punch down a keystone jack, and at a plug, as in pass-through vs standard RJ45.
Why a wiremap can miss a split pair
A split pair happens when one wire from each of two pairs is swapped at both ends. Every pin still connects to the same pin at the far end, so a simple DC continuity test passes. Electrically, though, the signal now travels on two conductors that are not twisted together.
The twist is what cancels crosstalk, so a split pair fails near end crosstalk badly and can cause errors or slow links. A common cause is following the wrong colour code on a jack. Testers that only check DC continuity cannot see it; you need a wiremapper that tests for split pairs, a qualification tester or a certifier.
How to test an Ethernet cable step by step
- Isolate the run and note its identifier.
- Connect the main unit at the panel and the remote at the outlet, using adapters for the link type you are testing.
- Run the wiremap and fix any open, short, reversed, crossed or split pair before going further.
- Check the length where the tester reports it and compare it with the drawings.
- Qualify or certify the run if the job calls for it, with the correct cable category selected on the tester.
- Save the result under the cable identifier, then retest anything you repair and keep both results.
A multimeter can confirm basic continuity on a cable, but it cannot detect split pairs, crosstalk or speed. Treat it as a last resort, not a test result.
PoE load testing
A cable that passes a wiremap can still fail to power a camera or access point. Resistance from a poor termination, a long run or undersized conductors drops voltage only when current flows. A PoE capable tester detects the switch or injector, reports the class offered and which pairs carry power, then places a load on the link to show what the device would actually receive.
Detection alone shows what the switch offers, not what survives the cable. Run a load test where devices draw higher power or a run is near its length limit. Sizing the source is covered in PoE budget for IP cameras and PoE injector vs PoE switch.
What test results to hand the client
On commercial work the specification sets the deliverable. A cabling specification from LSU Health Sciences Center, based on TIA-568-D, is a good model of what owners ask for:
- A result for every link, with a cable number matching the faceplate and patch panel, test date, length and wire map, plus the measured parameters for certified links.
- Tester name, maker, model, serial number, software version and calibration date.
- Marginal results marked as such; that project did not accept a marginal pass.
- For each repair, the problem, the corrective action, and both the failed and the passing test.
- Electronic and paper copies before acceptance.
On a small job, a spreadsheet of identifiers with pass results and the saved tester files is still worth handing over. It settles disputes later.
Where to find connectors and testing supplies
RJ45 plugs, keystone jacks and related parts are in the connectors category, and network cable is in bulk wire, both in the Wire, Cables and Connectors division. For tester options and pricing, ask your sales representative through the Bursma contact page or the Columbus, OH branch, or open a Bursma dealer account.
Frequently asked questions
Can you test an Ethernet cable with a multimeter?
Only for basic continuity, one conductor at a time. A multimeter cannot find split pairs or measure crosstalk, so it cannot tell you whether the cable will carry data reliably.
Is a link light enough to prove a cable works?
No. A link light shows that two devices negotiated a connection. It does not prove the run is wired correctly, supports the full speed or will power a PoE device under load.
Can a basic cable tester detect a split pair?
A tester that only checks DC continuity cannot. Use a wiremapper that tests for split pairs, a qualification tester or a certifier.
When do you need a cable certifier?
When the specification, the owner or a cabling manufacturer’s warranty requires category certification. For proving terminations on smaller jobs, a wiremap or qualification test is usually what is asked for.