How To Match Terminal Blocks To PLC And DCS Rack Requirements
Control system engineers often face a silent bottleneck: the terminal block. While it appears passive, its selection dictates signal integrity and thermal safety in factory automation. For main power feeds, potential distribution, and compact I/O, the TBNH, TBSH, and TBCH families each solve specific physical constraints. Misapplying them invites heat rise and intermittent errors. This guide compares electrical thresholds, mechanical limits, and installation trade-offs based on IEC 60947 and UL 1059 frameworks.
TBNH, TBSH, TBCH: Not Just Different Sizes
Engineers frequently treat feed-through, bridging, and ultra-dense blocks as interchangeable. In reality, their internal architecture varies fundamentally. The TBNH platform operates as a high-integrity feed-through conductor rated for 600 V AC, typically covering 15 A to 30 A loads. The TBSH series, however, is built around potential distribution. Its integrated jumper busbar removes the need for external shorting links. Meanwhile, the TBCH family tackles panel density, packing up to 32 connection points per vertical inch. Your first decision must be load type: power circuit or signal loop.
Electrical Ratings: Why 20% Headroom Is Mandatory
Precision starts with current and voltage data. TBNH units ship in 15 A, 20 A, and 30 A variants; all pass a 2500 V AC dielectric test for one minute. In contrast, the internal busbar geometry of TBSH limits it to 10 A continuous. For ultra-high density, TBCH single-contact capacity drops to 5 A. Field measurements show that once load exceeds 110 % of rating, temperature climbs non-linearly. We enforce a 20 % safety buffer on all power-related selections.
Conductor Sizes: Forcing The Wrong Wire Damages Reliability
Wiring flexibility directly affects installation speed. TBNH accepts 14 AWG to 8 AWG (stranded and solid) with a recommended screw torque of 4.5 lb‑in. TBSH targets signal circuits, supporting only 16 AWG to 12 AWG. TBCH saves real estate but restricts entries to 18 AWG fine wire. Forcing a 10 AWG cable into a TBCH port raises contact resistance by more than 50 %, and vibration resistance collapses.
Density Metrics: When TBCH Becomes Mandatory
When cabinet depth is locked, TBCH is the sole option. Standard TBNH mounts 12 positions per foot. TBSH improves this to 18 positions via pitch reduction. However, TBCH uses staggered columns to achieve 32 positions on the same rail. On a 24‑inch rack, this saves nearly 40 % of DIN rail space. For compact PLC racks inside modern machinery, this metric often decides the layout.
Fault Current: Power Circuits Must Stay On TBNH
System safety depends on behaviour during overloads. Third‑party benchmarks confirm TBNH withstands 1000 A prospective short‑circuit current for one second. Constrained by internal copper bridges, TBSH tolerance drops to 500 A. TBCH, engineered exclusively for signal isolation, fails above 100 A. We have witnessed TBCH disintegration in motor branches; avoid this mismatch entirely.
Equipotential Bridging: TBSH Cuts Labour By One‑Third
For multi‑circuit common supplies, TBSH significantly reduces wiring effort. Its one‑piece jumper channel requires no additional shorting links. One TBSH position expands to eight equipotential points via plug‑in bridges. TBNH, conversely, needs extra positions for potential distribution. This raises BOM cost and increases installation time by approximately 35 %. For sensor common negatives, TBSH is the intelligent shortcut.
Metallurgy: Silver Plating Matters In Harsh Environments
Base metal selection governs long‑term signal stability. Premium TBNH uses nickel‑plated brass; contact resistance stabilises below 0.5 mΩ. Some economy TBCH versions rely on thin phosphor bronze. After 1000 h at 85 % humidity, oxidation shifts resistance by 15 %. In chemical plants or coastal sites, we insist on silver‑plated variants. This experience‑based rule guarantees loop integrity.
Thermal Behaviour: High Density Needs Air Movement
Temperature rise correlates directly with lifespan. At 80 % rated current, TBNH housing rises only 18 K. Dense TBSH arrays impede airflow, resulting in 26 K rise. When ambient hits 55 °C, TBCH must derate to 3 A. Infrared scans show centre points in stacked TBCH run 7 °C hotter than edges. Forced cooling or generous spacing is non‑negotiable in high‑density layouts.
Marking Systems: Faded Labels Create Costly Rework
Large‑scale installations demand durable wire markers. TBNH features 8 mm square marking fields compatible with thermal transfer printing. TBSH uses side‑entry slots accepting only 5 mm narrow labels. TBCH top marking area is reduced by half. Handwritten stickers fade 60 % after three years. We strongly advise laser‑etched tags for long‑term asset management in DCS environments.
Vibration: Screw Torque Degradation In Moving Equipment
In robotic arm applications, 5 Hz to 500 Hz sweep tests reveal clear disparities. TBNH cage spring clamps sustain 20 N retention force; no momentary power loss occurs. TBCH, with higher self‑weight, exhibits fretting wear at resonance. Empirical data indicates TBCH screw torque degrades 22 % after 72 h of vibration. Anti‑loosening coatings are essential for moving assemblies.
Installation Economics: Speed Versus Rework Tolerance
Efficiency directly impacts project cost. With pre‑fabricated harnesses, TBSH push‑in termination averages 4.2 s per wire. TBNH screw fixing requires 6.8 s. For 10 000 termination points, TBSH saves 7.2 man‑hours. However, commissioning rework favours TBNH—its screw mechanism permits repeated locking without degradation. Assess your team’s wiring error rate before deciding.
Global Certifications: UL Recognition Is Not Optional
Export compliance demands rigorous scrutiny. TBNH series holds full UL 1059 and IEC 60947 accreditation; creepage distances satisfy 600 V reinforced insulation. Certain TBSH variants only carry CE Low Voltage Directive, limiting withstand to 300 V. TBCH units destined for North America must display the UL recognition mark. Non‑certified products invite project rejection and liability.
Total Cost Of Ownership: Cheap Blocks Hide Higher Expenses
Unit price alone is deceptive. TBNH costs approximately $1.20 per position—seemingly premium. Yet its 10‑year failure rate remains below 0.1 %. Low‑cost TBCH retails at $0.40, but specialised markers and elevated failure risk generate hidden expenses. Integrating labour, maintenance, and downtime, TBNH reduces total ownership cost by 18 % over lifecycle. This is frequently overlooked in bid evaluations.
Decision Matrix: Match Topology To Task
Synthesise your environment: for motor drive main circuits, choose TBNH. For multiple sensor common negatives, implement TBSH. For space‑critical I/O panels, deploy TBCH. Always amplify safety thresholds by 20 % as engineering margin.
Case Scenario: Automotive Assembly Line Retrofit
A recent project involved 12 PLC racks controlling welding robots. The original design used TBCH for all terminations. After six months, 15 % of sensor inputs exhibited intermittent faults. Thermal imaging confirmed centre‑row overheating. We retrofitted power feeds to TBNH, sensor commons to TBSH, and reserved TBCH only for dry contacts. Fault rate dropped to zero. This hybrid approach maximises both density and reliability.
Industry Perspective: Density Cannot Replace Power Physics
The trend toward miniaturisation challenges thermal physics. While TBCH pushes density boundaries, it cannot replace power blocks. We observe some OEMs attempting universal solutions; this often compromises safety. Our recommendation: maintain architectural separation. Leverage TBSH for intelligent bridging and TBNH for high‑energy paths. Active‑cooled terminal blocks may appear in the future, but today, physics dictates discipline.
Frequently Asked Questions (FAQ)
- Can I use TBCH for 24 V DC solenoid valves? Yes, if current is below 5 A per point and ambient temperature ≤45 °C. Derate by 20 % for grouped installation.
- Does TBSH support field‑side daisy‑chaining? Absolutely. Its integrated bridge busbar distributes common potential without external jumpers—ideal for 3‑wire sensor arrays.
- What torque driver setting for TBNH on 8 AWG? Set to 4.5 lb‑in (0.5 Nm). Over‑torque strips threads; under‑torque increases contact resistance.
- Are there hybrid blocks combining TBSH and TBCH features? Currently, no. Density and bridging capacity are inversely related. You must prioritise one attribute.
- How to verify plating quality on site? Use a portable thermocouple milli‑ohmmeter. Acceptable contact resistance is <1 mΩ for power, <5 mΩ for signal.
Contact Engineering Support: sales@nex-auto.com | +86 153 9242 9628 (WhatsApp)
Partner: NexAuto Technology Limited — Specialists in Industrial Connectivity & Automation Components.
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