Ground strap

Ground strap

A ground strap is a flexible electrical conductor, often a rectangular conductor. It consists of a braid of thin copper wires or a package of thin copper sheets. The ground strap is intended to establish the electrical connection between a device or a system and a reference potential . This is done either for personal protection, to discharge electrical charges, to ensure electromagnetic compatibility or for several of the reasons just mentioned.

Basics

Ground connector for equipotential bonding

Earth straps: ends with pressed sleeves (= additional contact resistance) and ends press-welded (below)

Ground straps have been used for a wide variety of tasks in electrical engineering for many decades. In automotive electronics , missing or defective ground straps are very often the cause of a malfunction in the electrical system. The importance of ground straps can be seen from the fact that the launch of the Ariane rocket with the first automated transfer vehicle (ATV) in March 2008 was postponed by one day in order to test the ground straps inside the spacecraft. Ground straps are available as so-called yard goods as well as pre-assembled. They are either made from bare copper wires or coated with other metals. They are used in electrical engineering:

• for directing great currents
• for dissipating EMC interference
• for dissipating static charges
• for radio interference suppression
• to compensate for potential differences

Source:

Properties of the ground strap

In order for a ground strap to perform the various tasks in electrical engineering, it must have certain properties:

• high current carrying capacity
• low ohmic resistance
• low impedance
• Mechanic solidity
• flexibility

Depending on the intended use, a ground strap must have some or all of these properties.

Current carrying capacity

Electric cables are often exposed to very high currents. In some cases, they not only have to master the normal currents that occur during operation , but also withstand short-circuit currents that are a multiple of the normal operating current. In vehicle electrics, ground straps are used as return conductors and must safely conduct the high currents of the starter (several hundred amps, depending on the vehicle type) to the battery. These ground straps must be capable of carrying current. The current carrying capacity of an electrical conductor is determined by its cross-section, its shape, the ambient temperature and the type of installation.

The cross section of a conductor is an essential factor for its current carrying capacity. The larger the cross-section, the greater the current-carrying capacity. However, this current carrying capacity is not proportional to the cross section. The current carrying capacity of rectangular conductors with standard cross-sections can be found in the tables of the VDE regulations. Since there are also many “intermediate sizes” for earthing straps in addition to the standard cross-sections, the current carrying capacity of the respective earthing strap can be seen from the manufacturer's documents.

The shape of a conductor is one of the decisive factors for its ability to dissipate heat. The larger its surface, the better its ability to dissipate heat. Rectangular conductors and therefore ground straps have a surface area up to 70% larger than round conductors of the same cross-section.

The ambient temperature is another factor that must be taken into account when determining the current carrying capacity of an electrical conductor. The conductor is heated by the current flowing in it. This heat has to be given off to the surrounding air. The temperature of the ambient air plays an important role here. If the conductor is warmer than the surrounding air, it can give off heat, otherwise it absorbs heat. The possible ambient temperature must be taken into account when dimensioning the ground strap.

The way a current-carrying conductor is laid is one of the decisive factors for its ability to dissipate heat and thus indirectly also for its current carrying capacity. This is a component that should not be neglected, especially in the case of ground straps that are loaded with a high continuous current. Heat can build up in closed housings if this heat is not dissipated sufficiently. So the temperature reaches z. B. in the engine compartment of a motor vehicle often values ​​of 70 degrees Celsius or more.

Ohmic resistance

Ground straps should have a low ohmic resistance . The ohmic resistance of a ground strap is made up of two partial resistances:

• Line resistance
• Transition resistance

The line resistance of a ground strap is influenced by the length, the cross section and the conductor material used. Ground straps should be as short as possible. Their cross-section should be as large as necessary, appropriate to the current load. Electric copper is predominantly used as conductor material. Sometimes so-called OFC copper is also used, this is an oxygen-free copper . Both types of copper have a purity of 99.995% and a conductivity of at least 58 m / Ω ​​* mm². This means that a 200 mm long ground strap with a 16 mm² cross section has a conductor resistance of just 216 µΩ. There are earth straps made of bare, tinned copper wires or copper wires with galvanic coatings of silver or nickel silver ( nickel - zinc- copper alloy ).

The transition resistance at the connection contacts of the ground strap is influenced by three factors:

• Skin resistance
• Tight resistance
• contact pressure

The skin resistance arises from the fact that metals form an oxide layer in the air . The thickness and electrical conductivity of this oxide layer depend on the material in question. Metals with very low skin resistance are alloys such as nickel silver or the precious metals gold and silver.

The tightness resistance depends on the surface of the material. Since no surface is absolutely smooth, there are more or less large contact areas at the contact points. The size of the contact areas is decisive for the level of the tight resistance. The following applies: the larger the sum of the individual contact areas, the smaller the resulting total resistance. The connection contacts of ground straps should therefore be as large as possible. A connection contact with a square area width * width is an optimal compromise.

The contact pressure has an influence on the tightness resistance and thus on the transition resistance. The contact pressure causes a slight deformation of the surface and thus an enlargement of the contact area. However, the contact pressure must not be too great, otherwise the ground strap will be damaged. This optimal contact pressure can be achieved with a special torque screwdriver . This prevents damage to the ground strap and prevents the disk from over-turning. At the same time, the optimal contact pressure ensures that there is no so-called loose contact .

Line impedance

Each electrical line (thus also a ground strap) can be assigned a capacitance per unit length and an inductivity per unit length. In addition, every electrical conductor has a specific conductor resistance (ohms per unit length) that must be taken into account. Due to the wide range of uses of a ground strap, the line impedance affects it in different ways.

In the case of direct current, only the line resistance, consisting of the conductor resistance and contact resistance, is effective.

In the case of low-frequency alternating current, the impedance ( Z ) of the ground strap is made up of line resistance ( ) and inductive reactance ( ). The ohmic resistance predominates up to a frequency of about 10 kHz. ${\displaystyle R_{ltg}}$ ${\displaystyle X_{L}}$

${\displaystyle Z_{ltg}={\sqrt {R^{2}+X_{L}^{2}}}}$

From a frequency of 10 kHz, in addition to the inductive reactance, the capacitive reactance of the parasitic power capacitances influences the impedance of the ground strap.

It now affects the reactance (reactance): ${\displaystyle Z_{L}=L^{\prime }/C^{\prime }}$

However, this influence only applies to so-called line pairs (i.e. forward and return conductors), e.g. B. with ground straps that are used as current straps, possibly noticeable.

From 100 kHz, the effect of the skin effect becomes very noticeable with a penetration depth of 0.21 mm. From around 1 GHz the penetration depth of 2.1 µm is so small that only the skin effect determines the level of the line impedance. The large surface of the ground strap has a positive effect here. Metallic coatings made of silver result in a low high-frequency resistance of the ground strap.

Above 1 MHz, the ohmic line resistance can be neglected because it is very small compared to the inductive reactance X _L caused by the line inductance . According to a rule of thumb, the self-inductance is 1 µH / m (length l >> diameter d) for a rectangular conductor. The size of the cross section of a rectangular conductor does not matter. However, this rule of thumb only applies to a long conductor in free space, whereby the conductor must be much longer than its diameter (at least 100 times as long). In addition, the conductor must be laid far enough away from other conductors that it is not influenced by interactions with the neighboring conductors.

At frequencies above 10 MHz, the inductance of rectangular conductors, including ground straps, is theoretically 20% lower than that of round conductors with the same cross-section. Ground straps should be as short as possible so that the inductive reactance caused by self-induction is as small as possible.

High frequency resistance

Due to the skin effect, from a frequency of around 1 GHz it is no longer important how large the cross-section of the conductor is, but only how large its surface is. Thin metal foils have the largest surface area, but these foils do not have sufficient strength and can therefore not be used.

The high-frequency resistance ( ) is a product of the direct current resistance ( R ) and the magnification factor ( n ) and can be calculated approximately: ${\displaystyle R_{\mathrm {HF} }}$

${\displaystyle R_{\mathrm {HF} }=R\times n}$

The magnification factor ( n ) is calculated as follows:

${\displaystyle n=k\times d\times {\overline {sf}}}$

the constant for copper is k = 3.75

With the skin frequency ( ) ${\displaystyle {\overline {sf}}}$

${\displaystyle {\overline {sf}}=F\times 10^{6}}$

This results in the formula:

${\displaystyle R_{\mathrm {HF} }=R\times k\times d\times {\overline {sf}}}$

In the high-frequency range, the length of the cable is decisive for the impedance. For this reason, ground straps for the high-frequency range should have a length-to-width ratio of 3: 1 to a maximum of 5: 1 in order to keep the inductance and thus ultimately the high-frequency resistance low.

Mechanic solidity

During operation, ground straps are exposed to a wide variety of mechanical loads. They are kinked, stretched, twisted, bent back and forth and guided over sharp edges. They have to endure all these burdens in part and still function for many years. The tensile strength of pure copper is 200 N / mm² in the soft state. In the case of work-hardened copper, the tensile strength is 360 N / mm².

flexibility

Ground straps are very often installed in narrow housings or have to connect machine or housing parts to one another without protruding far. Due to their shape, they have decisive advantages over round conductors. They are exposed to vibrations on rotating machines or are twisted in order to connect housing parts with one another in a low-resistance, conductive manner. Because of their design, ground straps are very flexible and have the necessary mechanical strength.

Types of ground straps

There are three types of ground straps:

• Ground straps with individual wires
• Ground straps with insulated individual wires
• Earth straps made from thin metal strips

Ground straps with a small cross-section (<4 mm²) are also known as ground strands. Ground straps , which are used to conduct large currents, are also known as current straps . These current bands are usually provided with an insulation made of shrink tubing .

Ground straps with single wires consist of many single wires with a single wire diameter of 0.05 mm to a maximum of 0.2 mm. Several individual wires are always joined together to form a strand and then the strands are stranded together to form a plait-like flat ribbon. The number and cross-section of the individual wires and the number of strands depend on the required cross-section of the ground strap. The ready-made ground straps are then made from this so-called yard goods.

Ground straps with isolated individual wires are made from so-called enamelled copper wire , which is covered with an insulating layer of insulating enamel. These ground straps are used to divert very high-frequency EMC interference . They are also known as grounding straps. Their electrical behavior is similar to that of HF litz wires .

Ground spring on vehicle axle

Ground straps made of thin metal lamellas are also known as expansion straps. When assembled, they often have the shape of a clamp with two lugs. They are used as ground straps on control cabinet doors. But they can also be used as power cords. In switchgear construction, expansion bands are used to decouple the vibrations of the transformer from the rail conductors. Common shapes are the V-shape (a central feature of the curved lamellas in the shape of an upside-down V) and the S-shape (2 × bent in V-shape). The latter also serves to bridge larger vertical distances.

So-called mass springs were previously installed in motor vehicles. These are meander-shaped ground straps that were built in to divert the electrostatic charge on the car tires.

Excerpt from table: Standard cross-sections of ground straps
cross-section	Wire diameter	Dimensions width * thickness
A in mm²	d in mm	(b × s) in mm²
6th	0.16	9 × 1
10	0.16	14 x 1.5
16	0.16	20 x 1.6
25th	0.16	22 × 2.5
35	0.16	25 × 3
50	0.16	33 x 3.2
70	0.16	40 x 3.5
95	0.16	50 × 4
120	0.16	55 x 4.5

Source:

Connection contacts

Ground strap, ends dip-tinned

Ground strap, with crimped end sleeves

The connection contacts on ground straps must be designed and created in such a way that they have a very low contact resistance. Normal bare copper strips are not so well suited because copper very quickly forms a poorly conductive oxide layer ( patina ) in the air .

Ground straps are more suitable:

• with dip tinned ends
• with crimped contact sleeves
• with pressure welded ends

Ground straps with dip-tinned ends do not tend to form an oxide layer that quickly, but their surface is very coarse, so that the connection contact is not optimal (narrow resistance). Ground straps with dip-tinned ends must be RoHS -compliant.

The connection contact is much better for ground straps with crimped contact sleeves . These contact sleeves (end sleeves) made of E-CU are galvanically coated with alloys such as nickel silver or precious metals such as gold and pressed on under high pressure without soldering. As a result, these connection contacts have very low transition resistances. By pressing the contact sleeves with optimal contact pressure, the air components are pressed out of the spaces between the strands and the connections in the contact sleeve become gas-tight. This means that no oxide layers can form inside the connection. If silver-plated strands are also used with ready-made ground straps, these ground straps are very well suited for diverting high-frequency currents.

Ground straps with pressure-welded ends offer minimal contact resistance due to the manufacturing process, which means that the ends are characterized by less heating and smoother thermal transitions. Another advantage is the non-use of additional river and Lotmitteln .

Advantages and disadvantages

advantages

• good heat dissipation due to the large surface
• low impedance
• great flexibility
• good flexibility
• universally applicable (except in cables)

disadvantage

• not usable in cables due to its shape
• Production is more complex than with round conductors

Areas of application

In motor vehicles

• as a return conductor (ground connection) on the battery
• to improve the conductivity of body parts and engine parts
• for bridging connections other than the aforementioned poorly conductive connections
• for discharging electrical charges
• to improve the shielding effect of shields
• for deriving HF interference

Source:

In many motor vehicles nowadays, for reasons of cost, round conductors with a corresponding cross section are installed instead of the ground straps.

For years, private individuals were offered so-called grounding strips (or anti-static belts ) in the automotive accessories trade. These should ensure electrical equipotential bonding through permanent contact with the road . Sales collapsed in the late 1990s when knowledge of the highly dubious benefit prevailed.

In control technology

• to improve the reference potential
• to improve impedance in functional earthing
• to divert EMC interference
• as flexible, low-resistance and low-impedance connections on the cabinet doors of control cabinets
• for creating a meshed earthing system
• to improve the impedance on metallic cable racks

In the hi-fi and computer area

• to improve the ground reference potential
• to bridge bad contacts
• for dissipating EMC interference
• for dissipating static charges

Source:

Statutory provisions and other regulations

• DIN 72 333 T3 battery terminals for starter batteries, ground strap, ground connector
• DIN 46 234 connections with solderless pressed cable lugs
• DIN 46 444 Braided copper strips made of flat-rolled copper hoses
• Law on the electromagnetic compatibility of equipment EMVG

See also

• Functional ground
• Wave impedance

Web links

• SPS ground straps (accessed August 26, 2016)
• Schmidthammer - Earthing straps - E-CU flat strands (accessed on August 26, 2016)
• EMS Power Connection (accessed August 26, 2016)
• Drahtwerk Wilhelm Gutmann - copper mesh belts (accessed on August 26, 2016)

Individual evidence

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2. ↑ ^{a b c} Herbert G. Mende: Funk-Entstörungs-Praxis. Franzis – Verlag, Munich 1978, ISBN 3-7723-0596-2 , pp. 77, 82–84, 89–91.
3. ↑ The first ATV is scheduled to start on March 9th at 4:59 a.m.CET. Retrieved August 26, 2016.
4. ↑ Wolfgang Müller: Electrical engineering specialist level, energy technology, specialist computing. Westermann Verlag, Braunschweig 1982, ISBN 3-14-201150-1 .
5. ^ Rudolf Hüppen, Dieter Korp: Auto electrics all types . Motorbuchverlag, Stuttgart 1968, ISBN 3-87943-059-4 , pp. 49-51.
6. ^ ^{A b} Günter Springer: Electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 , pp. 449-451.
7. ↑ Ismail Kasikci: planning of electrical systems. Theory - rules - practice. 2nd, updated and expanded edition. Springer Verlag, Berlin / Heidelberg 2015, ISBN 978-3-642-40969-1 .
8. ^ Hermann Franke, Kurt Juhl: Copper in electrical engineering - cables and wires . German Copper Institute eV; kupferinstitut.de (PDF; 650 kB) accessed on August 26, 2016
9. ↑ ^{a b} Hans Fischer: Materials in electrical engineering. 2nd, revised edition. Hanser Verlag, Munich / Vienna 1982, ISBN 3-446-13553-7 , p. 210.
10. ^ VGL - Allied Connectors: Flexible Copper & Aluminum Connections . ( alliedconnectors.com ( Memento of the original from September 10, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this note. Accessed August 26, 2016) 
11. ↑ Eduard Vinaricky (Ed.): Electrical contacts, materials and applications. Basics - Technologies - Test methods. 3. Edition. Springer Verlag, Berlin / Heidelberg 2016, ISBN 978-3-642-45426-4 , pp. 18-29.
12. ↑ Helmut Lindner, Edgar Balcke: Electrical Exercises Volume III Lines and Quadrupole. Vieweg Verlagsgesellschaft, 1977, ISBN 3-528-04064-5 , pp. 11-17.
13. ↑ Communication technology . Part 3: Wire-bound communication - line technology . Institute for the Development of Modern Teaching Methods eV, Bremen.
14. ↑ Rainer Thüringer: Impedance of electrical lines . University of Applied Sciences Giessen; wiki.fed.de  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 102 kB) accessed on August 26, 2016  
15. ^ Stefan Kloth, Hans Martin Dudenhausen: Electromagnetic compatibility. Expert-Verlag, ISBN 3-8169-1207-9 , pp. 3-7, 64-66.
16. ↑ T. Harriehausen: Impedance, admittance and reactance, The complex resistance . 2002, 2003; fh-weltebuettel.de ( Memento from September 23, 2010 in the Internet Archive ; PDF; 104 kB) accessed on August 26, 2016
17. ↑ SEW-Eurodrive (Ed.): Drive engineering practice . Volume 9: EMC in drive technology. fh-stralsund.de (PDF; 1.5 MB) accessed on August 26, 2016
18. ↑ Signal lines on printed circuit boards with controlled impedance . thm.de ; Retrieved August 26, 2016
19. ^ Curt Rint: Handbook for high frequency and electrical technicians. 13th edition. Volume 2. Hüthing and Pflaum Verlag, Heidelberg 1981, ISBN 3-7785-0699-4 , pp. 608-666.
20. ↑ Drive technology short seminar: EMC-equitable installation KEB ( Memento from June 20, 2016 in the Internet Archive )
21. ↑ Reyer Venhuizen: Guide to Network Quality - Earthing and Electromagnetic Compatibility Volume 6.1 Earthing with a system . German Copper Institute, Leonardo Power Quality Initiative; kupferinstitut.de (PDF) accessed on August 26, 2016
22. ↑ Zentralverband Elektrotechnik, Elektroindustrie eV: EMC - easily achieved . ifm.com (PDF) accessed on August 26, 2016
23. ↑ icotek swiss AG (Ed.): Massebänder - Earthing Connectors rofag.ch (PDF) accessed on August 26, 2016
24. ↑ ^{a b} Druseidt electric Spezialfabrik GmbH & Co. KG: Highly flexible mass, ground and power bands . Product information 02/2005 druseidt.de (PDF; 220 kB) accessed on August 26, 2016
25. ^ Rolf Gscheidle: Book of tables automotive technology. Verlag Europa – Lehrmittel, 2000, ISBN 3-8085-2133-3 .
26. ↑ Haarländer GmbH (ed.): Litz wires, braids, special products made from copper wires . Roth. donar.messe.de ( Memento of the original from August 26, 2016 in the Internet Archive ; PDF; 5.9 MB) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved August 26, 2016 
27. ↑ LEONI Draht GmbH (Ed.): Wires & Litzen for the cable industry . Weißenburg; cloudfront.net ( Memento of the original from August 26, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved August 26, 2016 
28. ↑ The Crimp ABC. KNIPEX plant C. Gustav Putsch KG; Retrieved August 26, 2016.
29. ↑ Cable glossary. WBT.
30. ↑ Jürgen Kasedorf, Richard Koch: Service primer for vehicle electrics. Vogel Buchverlag, ISBN 3-8023-1881-1 .
31. ↑ Useless tail . In: Die Zeit , No. 45/2001
32. ↑ German Society for EMC Technology eV (DEMVT): Guide to EMC-compliant control cabinet design . Rosenheim. ( online  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice .; accessed on August 26, 2016)