RJ45 Cable Tester UK: Wiremap, PoE and TDR Explained
An RJ45 cable tester checks that all eight wires in an Ethernet connector reach the correct pins, and a good one adds three things IT teams rely on: a readable wiremap that names the fault, tone-and-probe tracing with a port-flash to find an unlabelled run at the patch panel, and PoE and TDR checks. This guide explains each, with a focus on tracing cables back to the rack without unplugging live links.
If you have ever inherited a comms cabinet where not one of the patch leads is labelled, you know the job. A new starter needs their wall port patched into the right VLAN, but you cannot tell which panel port it lands on — and you cannot start unplugging leads during the working day without risking someone's call. An RJ45 cable tester with tracing turns that guesswork into a two-minute task.
Why RJ45 testing beats trusting link lights
Ethernet link lights lie. A cable with a crossed pair or a high-resistance crimp can negotiate a link at reduced speed — or show solid green while passing almost no usable data. An RJ45 cable tester removes the guesswork by testing each of the eight conductors and reporting a wiremap: pin 1 to pin 1, pin 2 to pin 2, through the T568A or T568B scheme used when crimping.
Reading RJ45 wiremap results
Run a wiremap and the display shows, per pair:
- OK: connects pin-to-pin correctly.
- Open: a wire is broken or unseated at one end — a bad crimp or pulled-back conductor.
- Short: two wires touch, often from nicked insulation.
- Cross: wires swapped, e.g. pin 1 to pin 3; a link may still appear.
- Split: a pair divided across non-standard pins — hard to spot without a proper tester.
Budget blink-light testers show green or red per pair. A unit like the Mcbazel Advanced 5-in-1 network tester shows the full wiremap on an LCD, names the exact fault type, and pairs with the tracing and PoE functions below.
Trace an unlabelled run to the patch panel
This is the feature that matters most in a messy cabinet. Two tools work together:
- Tone and probe: the main unit injects a tone onto a cable; the probe finds that cable in a bundle by the sound, so you can identify one lead among fifty. An adjustable-sensitivity probe with an earphone is worth having in a noisy comms room, where cheap toners cross-talk between adjacent cables.
- Port flash: connect at the wall port and the tester blinks the matching port's link light on the switch, so you can identify which switch port a wall socket lands on without unplugging anything. It works across 10/100/1000 switches.
Used together, you can map every wall port to its panel position in a single pass, label as you go, and verify the wiremap and length at the same time — turning an undocumented cabinet into a labelled one without downtime.
PoE detection on an RJ45 tester
Power over Ethernet feeds IP cameras, VoIP phones and access points. A continuity test cannot confirm PoE — the cable may be fine but the port unconfigured, or voltage may drop over a long run. PoE detection connects to a live port and reports the voltage present (5–60 V, covering 802.3af/at and non-standard). For the installer-focused detail, see our PoE cable tester guide.
TDR: finding faults without re-pulling
Time Domain Reflectometry sends a pulse and measures the reflection, converting it to distance so you know how many metres away a fault sits — invaluable when a run disappears into a riser or ceiling void. On the Mcbazel 5-in-1, TDR works on Cat5e, Cat6 and Cat6a from a single end. The full walkthrough is in our how to test an ethernet cable guide.
Choosing an RJ45 cable tester for UK work
| Feature | Budget tester | Mcbazel 5-in-1 |
|---|---|---|
| Wiremap (8-pin) | LED indicators | LCD with fault type |
| Tone trace + port flash | No | Yes (tone, probe, port flash) |
| TDR length/fault | No | Yes |
| PoE detection | No | Yes |
| Cat6a support | Sometimes | Yes |
| UK price | £10–£25 | £76.27 |
| Warranty | Varies | 12 months |
A tester built for the cabinet
For an in-house IT admin taming an undocumented network, the Mcbazel Advanced 5-in-1 RJ45 cable tester combines wiremap, port-flash tracing, PoE detection and TDR in one handheld with an adjustable-sensitivity probe. It is £76.27 inc. VAT (£63.56 ex. VAT), free UK next-day delivery, 12-month warranty, and 4.7/5 from 342 reviews — with IT users calling the port-flash "a lifesaver in messy server cabinets" for tracing unmarked wall ports back to the patch panel.
Untangling an unlabelled cabinet? Trace and verify every run without unplugging the office.
Honest limits worth knowing
Port-flash relies on the switch bringing the link up, so it identifies live-capable ports rather than a port that is administratively disabled. Tone tracing in a tightly packed bundle still takes care — sensitivity control and an earphone help, but adjacent cables can carry a faint tone. And this is a verifier, not a Cat6 certifier, so it will not print signed performance reports. For day-to-day IT fault-finding and documentation, those limits rarely get in the way.
Frequently asked questions
Can I identify which switch port a wall socket uses without unplugging?
Yes. Connect the tester at the wall port and use the port-flash function; the matching port's link light blinks on the switch. It works on 10/100/1000 switches, so you can map ports during working hours without disturbing live links.
Do I need different testers for Cat5e and Cat6?
No. RJ45 pinouts are identical across Cat5e, Cat6 and Cat6a. A tester that supports all three handles wiremap, TDR and PoE on any standard Ethernet cable.
Can an RJ45 tester check fibre?
No. Fibre needs an optical power meter or OTDR. Mcbazel testers are for copper twisted-pair Ethernet (RJ45/RJ11).