6.3 Grounding and 0 V

Every patch cable and every coaxial cable joins the 0 V of the two products it connects, so the products in a system share one 0 V whether or not anyone planned it. This clause makes that shared reference predictable: where 0 V runs in a row, across its power interfaces, how it meets protective earth, and how it reaches the next case.

In a row

A product shall connect every 0 V pin of its power headers, and the −V of its DC entry, to its 0 V (GND-001). The header's 0 V pins share the return current of all three bus rails, and a module that connects only some of them puts that current through fewer contacts. One 0 V gives a dual-source module one signal reference whichever entry is connected, and keeps a supply that takes PWR-DC12 and delivers the bus on one reference. An isolated power supply mounted on the product it powers joins its output 0 V to its input 0 V.

A bus board shall join the 0 V pins of every header to one 0 V that runs to the supply's 0 V terminal (GND-002).

EIS recommends keeping any panel-mounted part whose body touches the panel free of a connection to the module's 0 V; an insulated jack (7.1) meets that.

Return and reference

The 0 V of a row does two jobs, and EIS wires them differently.

• Return. Every module's current comes back to its supply through its bus board's 0 V and the board's 0 V conductor to that supply (GND-002). Each bus board has its own, a star from the supply, and never returns through another board: the drop on a return is that board's alone, and it does not move another board's reference.
• Reference. Every bus board's 0 V is the reference of every module on it, and a patch between two rows carries the difference between their references as signal. In a row with one supply, the board's copper and its return are the reference. With more than one supply, the returns end at different supplies, so where the bus boards of one enclosure are fed by more than one power supply, installation shall link their 0 V so that every bus board reaches, directly or through other boards' links, a bus board fed by each other supply, each 0 V link at most 2 mΩ terminal to terminal, and each bus board shall keep its own 0 V conductor to the supply that feeds it (ASM-009, 9.1). A chain of links along the rows does it; EIS recommends a grid, a link between every pair of neighbors, for three rows or more. The links hold every board at one potential whatever each return carries; the drop on a return then costs nothing but headroom.

A 0 V link is row wiring (6.1) on the boards' M4 0 V terminals; EIS recommends 12 AWG or heavier, or a braided strap or a bar, and the table in 6.1 gives the longest link of each gauge. A bus board may carry a 0 V strip, bare copper of its 0 V plane with a bus bar clamped on it, for the purpose (BUS-021, 6.5): its M4 holes, two rings on each, take its return and a link each way.

What a row's 0 V gains, in order: a bus board whose 0 V copper is a plane or a wide pour (6.5); 0 V links between the boards, which force one reference across supplies and across makers; a 0 V bar clamped on each board's bare 0 V strip, which takes the board's copper out of the path between its headers and its terminals; and the bar with the links.

One chassis tie

A system bonds its 0 V to the chassis, and the chassis to protective earth where it has one, at one point: the power entry of the case. A second bond gives mains-frequency and switching current a loop through 0 V, which shows as hum on audio and as slow horizontal bars on a picture.

A power supply publishes whether its 0 V output is connected to protective earth, and where (GND-003); a built-in supply publishes it in the enclosure's documentation. A product with a case DC entry publishes whether its 0 V is bonded to the chassis at the entry, and where (DC-014, 8.1). Mains safety, including the bonding of a metal case to protective earth, follows the safety standard the supply or case is certified to, not EIS.

EIS recommends that the product at the tie carry an M4 CHASSIS terminal, bonded to its 0 V or its earth in PWR-008's form and marked CHASSIS, so that straps to the case's other panels and metalwork land in one place; a DC entry panel is chassis where its screw heads press bare metal (Chassis continuity), and the terminal is for the parts that are not. A DC adapter whose output floats, the usual Class II desktop adapter, adds no second bond. EIS recommends that chassis ground carry only metalwork and screens, and bond to 0 V at the tie alone.

The mounting screws bond a front panel to the rails only where bare metal meets bare metal (Chassis continuity). With insulated jacks (7.1) the panel carries no signal return, and bonding the case metal to the supply's earth terminal, where it has one, is a shielding choice rather than a signal path.

Chassis continuity

Everything metal in a case other than the product at the tie is bonded to the tie only through the fasteners that hold the case together, and a fastener bonds two parts only where it presses conducting metal on conducting metal. Where a product makes a bond of a metal part to its 0 V, to its CHASSIS terminal, or to another metal part, and publishes it as a bond, the fastener shall press bare, plated, or conversion-coated metal, or a copper land clear of solder mask, not paint, powder coat, anodizing, or solder mask, and the bond shall measure at most 100 mΩ, and a bond of 0 V to the chassis (PWR-008) at most 10 mΩ (GND-004). The bonds it means are the ones a product publishes: a PWR-008 bond, a panel published as bonded to the rails, a strap, a link. The four-wire check is not the protective-bonding test of a mains product's safety standard, which a mains product still passes. An enclosure shall publish which of its metal parts, rails, inserts, sides, rear, and lid, are bonded to one another, and where its metalwork takes a bond to the row's 0 V (GND-005).

A joint either conducts, at a few milliohms, or it does not, at megohms, and no finish is conductive enough to carry a bond through it. The 100 mΩ figure is not a performance limit but a pass mark: the model in the eis-reference bonding page puts every metal-to-metal joint under 50 mΩ after years in a case, most under 10 mΩ, and every finished joint open. What a bond does for static, leakage, and interference depends on its length and width, not on its milliohms; what it needs is to exist.

EIS recommends:

• that a case bond every part a hand or a cable can reach: in the common wooden case with anodized rails and steel inserts the chassis is the inserts and the panels, and the rails float unless a bare pad or a strap joins them;
• that a part whose every path to the tie crosses a surface that does not conduct, or a loose fit such as an insert lying in an anodized groove, get a bare pad at one fastener, a conversion coating in place of anodize, or failing those a strap of PWR-008's form to the CHASSIS terminal or to a part already bonded, by the shortest route;
• that a module whose panel is to be chassis leave its front face bare under each mounting-screw head, a 7 mm disc at each hole on a coated panel or a finished copper land on an FR4 one; a coated panel with no bare spot is held by its screws and bonded by nothing;
• that a bond be checked four-wire at 1 A or more after assembly, from the CHASSIS terminal to the part.

Between cases

A patch cable or a VID-75 cable between two cases joins their 0 V through its sleeve or shell: a patch jack's sleeve is 0 V, and so is a VID-75 jack's shell (V75-002). A loop through two earth bonds shows as hum on audio and as slow-moving horizontal bars on a picture. EIS recommends:

• that the cases of one system take their 0 V from supplies that are all bonded to protective earth at the same outlet strip, or all floating;
• that a system spread over several cases, each with its own supply, join their bus boards' 0 V with a 0 V link, 0 V terminal to 0 V terminal, rather than through signal cables alone: 10 AWG or heavier, as short as the cases allow, and at most 2 mΩ where it can be, which on 10 AWG is about 0.44 m; and three or more cases as a grid rather than a chain, as Hinton Instruments' ZeroVolt link does with heavy cable and high-current connectors. ASM-009's 2 mΩ binds inside one enclosure, where a link that short can always be run.

Annex H turns the currents published under 6.1 into the heat of a row.