A fixed jib crosses fewer metres of cable than any other crane in the building and more joints than any of them. Four ways, one of them protective, and the one nobody lands.
A fixed jib crane moves power less than ten metres, from the wall to the hoist. It still crosses more joints than a 50 tonne overhead crane. A pillar jib that turns through a full circle needs a collector ring in the mast cap, a protective conductor stud inside a hollow tube, a hole in the base plate, a tight wire along the boom and a plug. Each one is a place where a machine that passed its load test in 2026 quietly stops being earthed in 2029, and none of them appear on the quotation.
The parts are cheap. A four way collector is a few hundred dollars. The tight wire kit that keeps the trolley cable out of the operator's face retails around 190 to 200 dollars. A 32 A blue plug costs less than lunch for two. What costs money is discovering three years later that the fourth wire was never landed.
I have opened enough masts to say this without hedging: nobody sells a jib crane with bad electrics. They sell it with electrics that were never finished. The man who bolts the mast to the foundation and the man who lands the supply are usually two different men, and the drawing that would join them up is a one page schematic at the back of a manual that nobody opens on site.
Because one of them rotates without limit and the other does not.
A 360 degree pillar jib has to pass current across a joint that turns forever. You cannot do that with a cable, so the cable stops at the mast cap and a stack of rings and brushes takes over. That assembly is the collector, and it is the only part on a jib that exists purely because the machine rotates. A 180 degree wall mounted jib never winds anything up. It swings out, it swings back, and a slack loop with a strain relief does the job for twenty years.
The earthing half is where manufacturers get blunt. Gorbel's free standing jib manual states that the crane cannot be utilised as a ground and that a separate ground wire is required, then gives the arithmetic: three phase systems need three conductors plus one ground wire. NEC 610.61 says the same thing at more length and adds that the trolley frame and bridge frame shall not be considered electrically grounded through the wheels and their tracks. OSHA published an interpretation letter in 2011 allowing a wheel path only where it stays permanent, continuous and effective. Paint, rust, dust and a shim pack are all things that end "permanent".
So on a rotating jib the protective conductor crosses the same joint as the phases. Four ways, not three. That single sentence explains most of the price difference between a collector ring and a terminal box.
| Arrangement | Rotation | Collector | Cable route | Where it fails |
|---|---|---|---|---|
| Wall bracket, 180 degree stop | 180 degrees, hard stop both ends | None | Single loop from bracket to hoist, dressed once | Jacket chafe at the bracket, and at the trolley where the loop is clamped |
| Pillar, 270 degree stop | 270 degrees, mechanical stops | None | Loop from the mast to the boom head, length set to the stop | Stop bolt shears, boom keeps turning, loop tears out of its gland |
| Pillar, bottom entry collector | 360 degrees continuous | 4 way, mounted on the pivot pin | Supply up through the base plate hole and the mast, collector in the cap | Protective conductor stud in the mast, water in the collector cover |
| Pillar, top entry collector | 360 degrees continuous | 4 to 6 way, bolted to the head plate | Supply dropped from overhead, flexible drop to the collector | Water running down the drop, and a drop loop that snags the boom |
The hardware is well documented, which is the frustrating part. HSI sells a bottom entry collector as a field kit: junction box pre-mounted on the mast, four conductor wires pre-run through the mast, collector dropped over the pivot pin and secured with set screws. Contrx draws the same arrangement on a pillar jib with a 4 inch hole in the base plate for the supply, a 4 ring bottom entry collector, a tight wire assembly along the boom and a stated rotation speed of 1/2 rpm. ABUS does it inside the pillar cap and describes the sequence in its manual: the supply is prepared from the mains switch to the slip ring, then the protective conductor is bolted to the pillar from the inside with an M5 by 20 fillister head screw at 5 Nm.
Five newton metres. That is a screwdriver, not a spanner. It is also the only thing bonding a length of steel tube to the supply earth on a machine that is about to be repainted by the buyer's own maintenance team.
Ways, mostly. Current is almost never the constraint.
A three phase jib needs three phases plus a protective conductor, so four ways is the floor. Single phase jibs do not need four, but four is what ships, because nobody wants to stock a three way assembly for one order. Once you add motorised rotation, or a push button station mounted on the boom, you are into six or seven ways: three phases, one protective conductor, and two to three control cores for rotation forward, reverse and common.
| Ways | What each way carries | Machine it suits | Risk if you get it wrong |
|---|---|---|---|
| 3 | Line, neutral, protective conductor | Single phase jib with manual rotation, no boom controls | Not a stock item, so it gets substituted with a 4 way and one ring sits idle |
| 4 | Three phases plus protective conductor, or line, neutral plus protective conductor | The standard jib, 0.25 to 5 t, manual or pendant controlled hoist | The fourth way is fitted but not wired. This is the most common defect we find |
| 6 | Three phases, protective conductor, two control cores | Motorised rotation with the push button on the boom | Control cores run outside the collector, looping round the mast and wearing through |
| 7 to 8 | Three phases, protective conductor, rotation forward, reverse, common, spare | Motorised rotation plus limit switches or a radio receiver on the boom | Spare ways get used for a signal the buyer adds later, and land on a phase |
The published envelope of an industrial slip ring built for crane service gives a sense of the margin. One type sold as a crane collector column offers 4, 8, 12, 24 or 36 rings, 30 A per ring, up to 600 V AC, insulation resistance above 200 megohm at 1000 V DC and a 1000 V AC hipot for 15 seconds. Put our worst case against that: a 3 tonne single phase jib drawing 21.7 A uses 72 percent of one ring. A 5 tonne three phase jib at 8.9 A uses 30 percent. So on a jib crane you never buy a bigger collector. You buy more ways, and you pay for the ones you leave unwired.
Less than most buyers assume, and the number barely moves between 3 tonnes and 5 tonnes because the hoist motor does not.
| Capacity | Installed load | Current at 230 V single phase | Current at 400 V three phase | IEC 60309 plug | Breaker, D curve |
|---|---|---|---|---|---|
| 0.125 t | 0.25 kW hoist, 0.12 kW trolley | 2.4 A | 0.8 A | 16 A blue, 16 A red | 10 A |
| 0.25 t | 0.5 kW hoist, 0.12 kW trolley | 4.0 A | 1.3 A | 16 A blue, 16 A red | 10 A |
| 0.5 t | 0.75 kW hoist, 0.2 kW trolley | 6.1 A | 2.0 A | 16 A blue, 16 A red | 10 A |
| 1 t | 1.5 kW hoist, 0.2 kW trolley | 10.9 A | 3.6 A | 16 A blue, 16 A red | 16 A |
| 2 t | 3.0 kW hoist, 0.2 kW trolley | 20.5 A | 6.8 A | 32 A blue, 16 A red | 25 to 32 A |
| 3 t | 3.0 kW hoist, 0.2 kW trolley | 21.7 A | 7.2 A | 32 A blue, 16 A red | 25 to 32 A |
| 5 t | 3.0 kW hoist, 0.4 kW trolley, 0.8 kW rotation | not offered | 8.9 A | 16 A red | 16 A |
Those figures assume a power factor of 0.80 and a motor efficiency of 0.85, which is realistic for a small chain hoist at rated load. Treat them as planning numbers rather than a substitute for the nameplate on the machine you actually bought.
The plug is where a buyer saves money by accident. IEC 60309 colours are not decoration: blue means 200 to 250 V, red means 380 to 415 V, yellow means 100 to 130 V, and the clock position of the earth pin makes a 230 V plug physically unable to enter a 400 V socket. A 2 tonne single phase jib at 20.5 A needs a 32 A blue plug and a 32 A socket. It very often arrives with a 16 A because a 16 A is what was in the box, and it runs for a year on light loads before someone lifts a full load in summer.
Same kilowatts, three times the current, and a factor of two instead of a factor of 1.73 in the volt drop formula. Single phase loses on both.
| 2 tonne jib, one way cable run | 10 m | 20 m | 30 m | 50 m |
|---|---|---|---|---|
| 230 V, 2.5 mm squared | 1.06 percent | 2.11 percent | 3.17 percent | 5.28 percent |
| 230 V, 4.0 mm squared | 0.66 percent | 1.31 percent | 1.97 percent | 3.29 percent |
| 230 V, 6.0 mm squared | 0.44 percent | 0.88 percent | 1.32 percent | 2.20 percent |
| 400 V, 2.5 mm squared | 0.17 percent | 0.35 percent | 0.52 percent | 0.87 percent |
| 400 V, 6.0 mm squared | 0.07 percent | 0.15 percent | 0.22 percent | 0.36 percent |
IEC 60364-5-52 Annex G is informative rather than mandatory, but the numbers everyone designs to are 3 percent for lighting and 5 percent for other final circuits, measured from the point of supply. On the table above, a 2.5 mm squared single phase run fails at 50 m and is uncomfortable at 30 m. The same machine on 400 V is under 1 percent on the same cable.
Steady state is the easy half. Motor starting is the half that decides whether the contactor holds. A direct on line chain hoist pulls six to eight times full load current for the fraction of a second it takes to accelerate. At 30 m on 2.5 mm squared that takes the single phase case from 3.2 percent to roughly 19 percent dip, and the 6 mm squared case to about 8 percent. Two things follow. First, size for the start, not for the run. Second, if the only supply available is single phase and it is 30 m away, the honest answer is to run three phase to the machine rather than a fat single phase cable.
The trade press says the same thing in fewer words. Guidance from one UK hoist supplier states that single phase hoists should always be run from an independent supply no more than 8 m from the hoist, because the current is high and cannot always be delivered otherwise. Eight metres is tight. It is also, in our experience, about where the nuisance calls start.
Whichever one matches what the machine puts on the wire, which depends on how the hoist is controlled, not on how big it is.
| Device type | What it senses | Behaviour on a jib | Verdict |
|---|---|---|---|
| AC | Alternating residual current only | Blind to the half wave DC from a brake rectifier and blind to a drive | Wrong device for any hoist. Still sold, still fitted |
| A | Alternating plus pulsating DC | Sees a contactor controlled hoist with a half wave brake rectifier. Does not see a drive | Correct for the majority of small jibs |
| F | As type A plus high frequency up to 1 kHz | Fewer nuisance trips where a drive shares the circuit | Reasonable compromise, less common in stock |
| B | Alternating, pulsating and smooth DC | The only type that sees a variable frequency hoist. Also the type that stops an upstream AC device being blinded | Mandatory in our view from the moment a drive is fitted |
Schneider builds a device specifically for this problem: a type B residual current breaker with what they call super-immunised sensing, aimed at cranes with speed drives and tested over long cable lengths, with the explicit note that the protection chain has to be coordinated so a downstream device does not blind the one upstream. That last point is the one people miss. Two RCDs in series, one type A upstream and one type B downstream, can leave the upstream one unable to trip at all.
If the machine is contactor controlled and trips only occasionally, do not buy a device yet. Clamp the standing leakage on the circuit with the crane isolated, then again with it energised, and compare. Half the time the culprit is a bank of LED drivers on the same circuit, and no RCD type fixes a shared circuit.
Inside the mast if you follow the German pattern, on the wall if you follow the American one, and either can be compliant.
ABUS fits the mains switch into the pillar and wires the slip ring from it, which keeps everything on the machine and makes the crane a single package. Gorbel and Contrx draw the isolator as a wall device and the crane as a plug-in appliance. Both work. Neither is what the regulations actually say, because the regulations talk about the disconnecting means being capable of being locked in the open position, being within view of the supply, and being reachable from the operating station if it is not readily accessible.
OSHA 29 CFR 1910.306(b)(2) is the text most buyers end up arguing about. It requires a motor circuit switch, moulded case switch or circuit breaker in the leads on all cranes and monorail hoists, lockable open. Then subparagraph (iv) allows that disconnecting means to be omitted where three things are all true: the unit is controlled from floor level, it is within view of the power supply disconnecting means, and no fixed work platform has been provided for servicing it. Most 0.5 to 2 tonne jibs with pendant control satisfy all three. The same subparagraph also requires a means at the operating station to open the power circuit to all motors where the disconnecting means is not readily accessible from that station. A pendant with a main contactor does that. A plug does not.
There is a second requirement that gets forgotten because it is about space rather than electricity. The working space in front of live parts that may need servicing while alive has to be at least 762 mm, and where controls sit in an enclosure the door must open at least 90 degrees or be removable. A switch buried inside a 200 mm mast with a hinged cover and a padlock has to be checked against both.
| Isolating arrangement | Compliant | serviceable | Notes from site |
|---|---|---|---|
| Lockable switch on the wall beside the jib | Yes | Best | Add a 32 A socket next to it and the whole machine becomes a plug-in appliance |
| Switch inside the mast | Yes, if the door opens 90 degrees | Poor | Needs a key and a torch. LOTO tags get left inside and forgotten |
| Plug and socket only | Only under the floor-controlled exception | Fair | Pulling a plug at head height to change a brush is how people get hurt |
| Pendant main contactor | Not a substitute | n/a | It satisfies the operating station requirement. It does not isolate for maintenance |
It goes down a tube, past a raw edge, and into a foundation that may have been poured before anyone decided where the supply would come from.
The drawings are specific about the hole. Contrx puts a 4 inch (102 mm) opening in the base plate for the supply and reminds the installer to allow for power conduit when pouring the footing. HSI pre-runs four conductors through the mast and mounts the junction box on it. Both approaches mean the riser cable is fixed at two points with a long free length between them, which is fine until somebody pulls it tight to make it look neat.
Bending is where cables die. LAPP's assembly guidance for crane cable gives the minimum bending diameter as ten times the cable outer diameter for cables up to 21.5 mm, 12.5 times above that, and 15 times for the versions with a polyurethane jacket. A 12 mm outside diameter riser needs a 120 mm bend diameter at ten times, 180 mm at fifteen. A 4 inch hole has room. A 25 mm cable gland does not, and that is what usually gets fitted because it was on the shelf.
| Riser cable outside diameter | Minimum bend diameter at 10 times | At 12.5 times | At 15 times | Practical consequence |
|---|---|---|---|---|
| 8 mm, 4 core 1.5 mm squared | 80 mm | 100 mm | 120 mm | Fits an M20 gland and a 4 inch base hole with room to spare |
| 12 mm, 4 core 2.5 mm squared | 120 mm | 150 mm | 180 mm | M25 gland is fine, but the cable must not be clamped against the hole edge |
| 17 mm, 4 core 6 mm squared | 170 mm | 212 mm | 255 mm | M32 gland, and the bend now has to happen inside the mast, not at the plate |
The bonding point matters more than the bend. ABUS specifies an M5 by 20 fillister head screw at 5 Nm to bolt the protective conductor to the inside of the pillar. That is a small fastener into a thin wall, and it is doing the job that a 25 mm earth bar does on a factory floor. Two things kill it: over-torquing, which strips the thread, and paint, which is applied by the buyer's team at the next shutdown. We ask for a shakeproof washer and a dab of touch-up paint around the stud, not over it.
Not in any timescale that matters. That is not the same as saying it never fails.
Slip ring manufacturers publish preventive maintenance intervals in revolutions, not hours, which is the honest way to do it. One industrial slip ring maker gives the following baseline for flange mount units, before accumulated brush dust has to be cleaned out.
| Operating speed | Revolutions before cleaning, up to 7.5 A per ring | Revolutions before cleaning, above 7.5 A per ring | Years on a 0.5 rpm jib at 100 lifts a day |
|---|---|---|---|
| 1 to 5 rpm | 5,000,000 plus | 4,000,000 plus | 640 |
| 6 to 15 rpm | 4,000,000 | 3,200,000 | 512 |
| 16 to 30 rpm | 3,200,000 | 2,880,000 | 410 |
| 31 to 50 rpm | 2,880,000 | 2,590,000 | 369 |
The years column is our arithmetic, not the manufacturer's. A jib turning at half a revolution per minute, swung through a quarter turn per lift, 100 lifts a day, 250 days a year, accumulates about 6,250 revolutions annually. Four million revolutions is six hundred and forty years. Even at a thousand lifts a day it is sixty-four years, which is longer than the building.
Then the same manufacturer tells you to inspect every 200 to 400 hours of operation, and to replace a brush when the distance from the top of the insulator to the bottom of the brush spring reaches 0.093 inch, which is 2.36 mm. Both instructions are correct. The wear table says you will never wear the brushes out, and the inspection interval says look at them four or five times a year. The reason is that the inspection is not looking for wear. It is looking for dirt.
Brush dust is conductive. It builds up inside the cover, it bridges between rings, and eventually the assembly arcs over. Moisture does the same thing faster: Conductix-Wampfler's slip ring manual calls moisture a major cause of deterioration and puts the first inspection shortly after commissioning, then every 200 to 400 hours. Where two brushes run on a ring at under 2,500 surface feet per minute, the dynamic contact resistance should stay below 10 milliohm, and once the brush has worn past its spring's ability to press it, that number goes up without anyone noticing.
So the honest maintenance position on a jib collector is this. Do not budget for replacement. Budget for a two hour clean every quarter, and buy the gasket that the cover came with, because the cover is where the water gets in.
The collector, almost never. Everything around it, routinely.
| First year electrical fault | Share of our service records | Root cause in one line | Prevention |
|---|---|---|---|
| Protective conductor not bonded at the mast or cap | 34 percent | Earth landed on the base plate, or the stud painted over | Measure at handover, write the reading on a label inside the door |
| Riser cable chafed at the base plate hole | 21 percent | Cable pulled tight over a raw edge, no gland, no grommet | Gland plus 100 mm of slack inside the mast |
| Plug or socket under-rated | 16 percent | 16 A blue on a 2 to 3 tonne single phase machine | Specify the plug on the order, not on site |
| RCD type mismatch | 12 percent | Type AC on a drive or on a brake rectifier | State the type in the RFQ; type B wherever a drive is fitted |
| Collector water or brush dust | 9 percent | Cover gasket missing, machine washed down, or no quarterly clean | Keep the gasket, clean with dry air and isopropyl alcohol |
| Rotation stop sheared, loop torn | 5 percent | Forklift or crane hook caught the boom and pushed past the stop | Replaceable shear bolt rather than a welded lug |
| Control transformer or 48 V circuit fault | 3 percent | Water in the push button station, or a pinched control core | IP65 station, strain relief, and control cores inside the collector |
Those percentages come from our own service records on jib installations, not from a published survey, and I would not defend the third decimal place. The ordering is what matters. Three of the top four causes are installation errors that cost nothing to avoid and everything to find, because none of them stop the machine from working. A jib with a 3 ohm earth path lifts exactly as well as one with 0.05 ohm. It just kills somebody differently.
Less than most people think, and more than the part price suggests, because what you are really buying is a joint you will have to look after.
| Item | Indicative cost | What it buys | What it costs later |
|---|---|---|---|
| 4 way bottom entry collector | USD 350 to 900 | Continuous rotation with the supply entering through the base | Two hours of cleaning a year, one gasket |
| 4 way top entry collector | USD 350 to 900 | Same, fed from an overhead drop | Water tracking down the drop into the cover |
| 6 way collector with motorised rotation | USD 500 to 1,300 | Rotation control on the boom, no separate loop | Two more ways to mis-wire, two more to clean |
| Tight wire kit, 25 ft boom | USD 187 to 202 retail | Keeps the trolley cable off the floor and out of the load path | Re-tension once, then a cable every five to eight years |
| Lockable wall isolator, 32 A | USD 60 to 250 | A tag-out point at eye level | Nothing. This is the cheapest safety item on the machine |
| Type B RCD, 40 A four pole | USD 200 to 600 | Protection that a drive cannot blind | Panel space, and a device that must be tested twice a year |
| 32 A blue plug and interlocked socket | USD 40 to 150 | 100 percent of the current, in the correct colour | A plug body every few years in a welding shop |
The published retail prices are the tight wire kits; the rest are our own quotation ranges across 2025 and 2026 projects and should be treated as a sanity check rather than a price list. What the table makes clear is that the electrical completeness of a jib is a four figure decision in total. It is never the reason a project goes over budget, and it is frequently the reason a machine is unsafe two years after handover.
Whenever the process does not actually need a full circle.
A 180 degree wall mounted jib from Kundel is sold with what they call a festooning free system, and the phrase is honest: with a 180 degree swing there is nothing to manage, the cable loops once and returns. That is the most electrically reliable arrangement on the market and it costs nothing extra. A pillar jib with a 270 degree mechanical stop deletes a joint the same way. You give up ninety degrees of circle and you delete a wear item, a cleaning task, a water trap and a bonding point.
So the question to ask before ordering a 4 way collector is not "can we have 360 degrees" but "does any station in this cell sit behind the mast". In a welding bay feeding two fixtures placed 120 degrees apart, the answer is usually yes and you need the collector. In a single press tending cell where the operator loads from one side and the press is on the other, 180 degrees does the same job with one less joint.
If the process changes later, retrofitting is possible but not elegant. Collector assemblies are sold separately, the mast cap usually has to be drilled or replaced, and the four conductors have to be pulled through a mast that is already standing with a hoist on it. It is a day of work and a scaffold. Decide at order stage, when it is a line on a drawing.
Readings, not just drawings. The readings are what makes the annual inspection a comparison instead of a guess.
| Measurement | Instrument | Pass figure we use | How it drifts |
|---|---|---|---|
| Protective conductor, frame to building earth bar | Low resistance ohmmeter with at least 10 A test current | 0.1 ohm or less. Above 1 ohm, investigate before running | Paint over the stud, loose M5 screw, corroded ring |
| Insulation resistance, ring to ring | 500 V or 1000 V insulation tester | Above 200 megohm when new, per the published figure | Brush dust, moisture, washdown chemicals |
| Standing earth leakage on the circuit | Leakage clamp meter | Record with the crane off and on. Compare year to year | Other equipment added to the same circuit |
| Voltage at hoist terminals under load | Multimeter, held during a lift | Within 5 percent of nominal | Longer cable run, higher ambient, extra load on the board |
| Cable condition at both stops | Hands and a torch, boom swung to each stop | No tension, no kink, jacket intact | Someone re-clamps the cable to make it look neat |
| Brush wear gap | Feeler gauge | Replace at 2.36 mm (0.093 inch) from insulator top to brush spring | Very slowly, which is why nobody checks it |
| Plug and socket temperature after an hour | Infrared thermometer | Warm is normal. Too hot to hold is a contact problem | Worn contacts, undersized pins, a plug used as a switch |
Write those numbers on a label and stick it inside the mast door. It takes four minutes at handover and it converts every future inspection from an opinion into an arithmetic problem. We do this on the pillar and wall mounted jib cranes we ship, and the label outlasts the commissioning engineer, which is the point.
Send these as questions rather than requirements and you will get answers you can compare. Send them as requirements and you will get a price.
Question four is the one that saves money. Buyers ask about ways and price, and forget to ask whether the fourth way is a functional conductor or a spare. It is a two word clarification that decides whether the machine is earthed or merely looks earthed.
No, and the manufacturers are unusually direct about it. Gorbel's manual states that the crane cannot be utilised as a ground and that a separate ground wire is required, with three phase systems needing three conductors plus one ground wire. NEC 610.61 says the trolley frame and bridge frame shall not be considered electrically grounded through the wheels and their tracks, and requires a separate bonding conductor. On a rotating jib the practical consequence is that the protective conductor has to cross the collector, so you need four ways and you need them all landed.
No, and the machine will be more reliable without one. A 180 degree wall mount swings out and back, so a dressed cable loop with a strain relief handles the movement and there is no ring or brush to clean, no cover to leak and no fourth conductor to forget. Some manufacturers market this explicitly, describing a festooning free system. The same applies to a pillar jib fitted with a 270 degree mechanical stop. Take the collector when the process genuinely needs the full circle.
Depends on the distance, and it is closer to the limit than people expect. A 2 tonne single phase jib draws about 20.5 A at 230 V, so it needs a 32 A circuit and a 32 A blue plug rather than the 16 A one usually supplied. At 30 m of 2.5 mm squared you are at 3.2 percent drop, which is inside the 5 percent design figure until the motor starts and the dip goes to roughly 19 percent. Use 6 mm squared at that distance, or bring three phase to the machine, which sits under 1 percent on the same cable. One UK hoist supplier goes further and advises that single phase hoists run from an independent supply no more than 8 m away.
Usually yes, and it takes a day plus access equipment. The parts are sold separately, the mast cap usually needs drilling or replacing, and four conductors have to be pulled through a mast that already has a hoist hanging off it. Budget a collector, a new cover gasket, a gland and the labour to re-commission the earth. If the crane is still on order, this is a line on a drawing instead.
Clean it quarterly and inspect every 200 to 400 operating hours, which is what the slip ring manufacturers ask for. Do not expect to replace brushes: at half a revolution per minute the published four million revolution interval before cleaning exceeds six hundred years of typical jib use, so wear is not the failure mode. Dirt, moisture and a loose protective conductor are. Keep the cover gasket, use dry compressed air and isopropyl alcohol rather than a solvent, and never point a high pressure air gun at the brushes themselves, because they have very little transverse strength.
Resistance from the crane frame to the building earth bar, with a low resistance ohmmeter that injects at least 10 A. Target 0.1 ohm or less, and treat anything above 1 ohm as a fault rather than a reading. A multimeter will tell you the circuit is continuous and tell you nothing about whether it will carry fault current, because it cannot see a 3 ohm path through a shim and a coat of primer. That is the same reading a 1 tonne jib in a mould shop in Vietnam failed in August, four months after commissioning, with an earth landed neatly on a painted base plate. The full electrical and mechanical specification for our jib crane range lists the handover values, and they are the values to re-take every year.
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