For electrical-installation concrete work, the practical default is an internal (poker) vibrator running 8,000-14,000 vibrations per minute, with head diameter chosen to the pour depth and rebar spacing [S1].
Electrical conduits, duct banks, ground-grid pads, transformer plinths and switchgear foundations all introduce the same constraint: dense reinforcement and embedded metalwork that blocks internal access in places, plus the need for low-voltage site distribution that the vibrator must plug into safely [S1][S5]. For related product context on the wider category, see the concrete vibrator encyclopedia entry and the electrical installation reference.
Internal poker vibrator: the working baseline for conduit and plinth pours
Internal vibrators account for the majority of on-site concrete consolidation work, with head diameters of 1 inch to 2.5 inches (or larger) and shaft lengths from 3 ft to 21 ft; effective radius of action is 3-10 inches depending on head size [S1]. The matching rule of thumb from the same source: a 1 inch head suits pours up to 6 inches deep, while a 2 inch head suits pours up to 18 inches deep, which covers most duct-bank and pad pours around an electrical room.
For duct banks and heavily reinforced transformer foundations, a 1.5-2.0 inch head at 10,000-12,000 VPM is the common operating point, and a 200 Hz high-frequency electric drive (about 12,000 VPM) gives finer, faster compaction than 50 Hz standard units [S1][S4]. Battery (cordless) and mains-electric high-frequency models are both available; battery tools are increasingly specified on indoor fit-out where diesel or 230 V leads are restricted [S4]. If you need a quick pointer for the broader mechanical side of the same pour, the vibrating poker guide walks through adjacent selection logic for a different trade.
External form vibrator: when rebar or form geometry blocks internal access
External (form-mounted) vibrators generate 500-10,000 lbf of centrifugal force through the formwork wall and are the right call when internal access is limited by congested rebar, thin walls, or precast moulds typical of cable-tray support columns and switchgear housings [S1][S3]. Vibtec specifies high-frequency electric motors at 6,000 RPM with single or three-phase supply, plus pneumatic alternatives (VE, VAQ) where electric supply is hazardous or unavailable, and quick-release clamps for steel or wood forms [S5].
Mounting matters as much as motor size: clamp-on, weld-on or bolt-on brackets sized to the form material, and synchronous control of multiple units from a single DOL or variable-speed controller, are standard practice on larger electrical-installation pours (e.g. long cable-duct walls) [S5]. For site power in classified or damp areas, suppliers typically pair these drives with DOL starters and frequency converters; the wider selection picture for electrical automation gear on a vibrator rig is laid out in the encyclopedia. When the structure itself is steel rather than concrete, the steel-construction vibrator selection note covers the overlap.
Surface screed and vibrating table: thin slabs, panels and precast
Surface vibrators (vibrating screeds and pan units) consolidate only the top 4-6 inches and are suited to floor slabs, pavements, and the thin topping slabs often placed over conduit in electrical rooms; vibrating tables are used for precast panels, small utility boxes, and pull-pit lids cast off-site [S1][S3]. Bull-float, truss and beam screeds differ in profile and beam length, but all rely on a 50 Hz or high-frequency drive matched to the slab thickness and the desired surface finish [S4].
For thin slabs (under 100 mm) the screed becomes the primary compactor because a poker would over-vibrate and segregate the mix; for a 150-200 mm slab on a conduit grid, a screed first pass followed by a 1.5 inch poker at the edges is the typical sequence [S3][S4]. Power-float and trowel timing comes after vibration, not instead of it, and is governed by the bleed-water window, not by the vibrator spec [S4].
Selection criteria compared across the four types
Across internal, external, surface, and table vibrators, the four criteria that actually drive the buy are: pour depth, rebar congestion, power-source availability, and required surface finish [S1][S3][S6]. The table below lines them up so an electrical-installation buyer can match the tool to the task.
Type selection matrix for electrical-installation concrete work [S1][S3][S5][S6]:
Internal poker: 1-2.5 inch head, 8,000-14,000 VPM, mains 230 V / battery / petrol, suits conduits, plinths, duct banks, and ground-grid pads with open access. External form: 500-10,000 lbf centrifugal force, 6,000 RPM / 2-8 pole, single or 3-phase electric plus pneumatic options, suits congested rebar in thin walls, precast cable-duct sections, and architectural electrical-room walls. Surface screed: 4-6 inch effective depth, 50 Hz mains or HF drive, suits indoor floor slabs over conduit trays. Vibrating table: small precast lids, pull-pit covers, off-site utility boxes, and ground-bar supports; the table is sized to the mould footprint rather than the pour.
For electrical-installation work specifically, the rebar-congestion criterion tilts the decision toward external form vibrators on switchgear walls and toward internal pokers on transformer plinths, where the rebar mat is heavy but the pour depth (12-24 inches) still allows insertion. Selection must also respect the electrical safety constraints of the work area, since a wet, reinforced pour next to a live switchboard is the worst place for a 110 V tool with a cracked lead.
Power source and electrical safety on site
Electric vibrator drives are typically 230 V single-phase or 400 V three-phase at 50 Hz, with high-frequency units running on an integrated inverter that rectifies to about 200 Hz (12,000 VPM) for finer compaction [S4][S5]. Pneumatic vibrators (VE, VAQ series at 8-55 cm sizes) are a safer choice in confined electrical rooms, explosive-atmosphere zones near transformers, or anywhere a stray electric spark is unacceptable [S5].
For classified or damp areas, the practical move is to specify pneumatic drives through a lubricated air line, with a 24 V or battery pilot circuit for the controller, rather than a 230 V tool on a wet slab. Darda's own internal-vibrator guidance ties the choice to component thickness, reinforcement ratio, concrete consistency, and required surface quality, which is the same criteria an electrical-installation buyer should apply when calling out a tool on a substation or data-hall pour [S6].
When not to vibrate: failure modes and limits
Over-vibration segregates the mix (heavy aggregate sinks, water and cement rise), producing weak laitance and sand streaks on formed faces, while under-vibration leaves the 5-20% entrapped air in place that drops compressive strength by 5-15% versus properly vibrated concrete [S1]. The repair cost of bug holes alone runs $15-50+ per square foot, and honeycombing around embedded conduits is a documented cause of partial discharge tracking in service, because voids concentrate the field at the rebar and duct interface [S1].
Do not vibrate concrete that has begun initial set, and do not rely on a surface screed as the sole compactor on any pour over 150 mm thick. If a poker cannot be inserted without touching rebar or conduit, the right answer is an external form vibrator on properly clamped brackets, not a smaller poker forced into the gap [S1][S5]. Sourcing for explosion-proof electrical accessories in classified zones, and for measurement instruments that verify vibration amplitude, is a separate procurement line item from the vibrator itself.
Trackable signals to watch over the next planning cycle: supplier extensions of the 200 Hz integrated-inverter poker range, and the appearance of more 48 V battery-drive pokers rated for indoor electrical-room use, both of which were flagged as active product directions in the 2026 vendor literature [S4][S5].