Grounding concept considerations and recommendations for 400VDC distribution system

Keiichi Hirose R&D Headquarters NTT Facilities, Inc.

2-13-1, Kitaotsuka, Toshima-ku,

Tokyo, 170-0004 JAPAN

hirose36@ntt-f.co.jp

Toshimitsu Tanaka and Tadatoshi Babasaki NTT Energy and Environment Systems Laboratories

NIPPON TELEGRAPH AND TELEPHONE CORPORATION

3-9-11 Midori-cho, Musashino-shi, Tokyo, 180-8585 JAPAN

tanaka.toshimitsu@lab.ntt.co.jp

babasaki.tadatoshi@lab.ntt.co.jp

Sylvain Person and Olivier Foucault France Telecom RD LANNION

RESA/DEAN, 2 avenue Pierre Marzin

22307 LANNI ON CEDEX, FRANCE

sylvain.person@orange-ftgrpoup.com

olivier.foucault@orange-ftgrpoup.com

BJSonnenberg Business Development

Emerson Network Power

1716 Tealwood PL

Raleigh, NC 27615 USA

BJ.Sonnenberg@Emerson.com

Marek Szpek Business Development

Emerson Network Power

Textilgatan 29

12007 Stockholm, SWEDEN

Marek.Szpek@emerson.com

Abstract-400VDC is becoming a new voltage interface for telecommunication buildings and data centers as well as for ICT equipment. 400VDC interface covers applications for up to 400VDC with a typical nominal voltage of 380VDC. Key drivers for utilizing a higher DC voltage are overall efficiency and reliability gains vs present solutions. However, introduction of a new 400VDC interface brings up some questions about personnel safety and implementation of system grounding. Although DC is considered safer than AC, till now datacom operators have been hesitant to deploy 400VDC because it's not as commonly used and well understood as AC. This paper addresses 400VDC grounding issues and recommends high resistance midpoint grounding concept, as an effective way to provide safe operation of 400VDC distribution systems.

Today Europe, North America (NA), and Japan tele­com/datacom installations employ slightly different grounding concepts, practices, codes, or regulations. This paper also reviews different grounding models and their implementation praxis in Europe, NA, and Japan. Benefits and the disadvantages of different grounding concepts are discussed and explanation of the reasons for selection of high resistance midpoint grounding as a preferable model for 400VDC distribution system is provided.

Index Terms-400VDC, Power system, grounding, Bonding, Data center.

I. INTRODUCTION

Increasing equipment rack densities, facility space con­straints, drive for efficiency improvements and simplification of an interface with renewable energy resources bring attention to higher voltage DC systems as an attractive alternative to present electrical power distribution in datacom applications [1], [2]. Additional benefits of higher voltage distribution include elimination of harmonics , no need for load balancing between the phases like in case of three phase AC systems, simplification of distribution network, wiring size reduction, space savings etc. Many higher voltage DC systems were

978-1-4577-1250-0/111$26.00 ©2011 IEEE

and are deployed globally at univerSities, laboratories and commercially, and are under continuous study. Results from the field trials to date confirm feasibility of deployment and significant efficiency gains vs. existing AC UPS solutions.

A typical 400VDC system uses nominally 380VDC voltage in normal operation and has a simple configuration as shown in Fig. 1 [3]. Floor space and the distribution system power cables sizes in the system are significantly improved in comparison with other solutions (AC or DC). Moreover, the total amount of power loss reduction is estimated to be 7-10% vs. best in class AC solutions. The number of 380VDC trials has significantly increased during last year and the interest for new DC interface is extending from data centers to commercial buildings for applications with lighting and air conditioning.

Selection of a proper grounding method is a major con­sideration for a safe and stable performance of any electrical distribution system. Personnel safety factor is a fundamental consideration for systems employing elevated voltages (over

DC System (48 V)

Fig. I. Advantages of 380V direct current power distribution system.

60V SELV range) both AC and DC. Other factors include EMI immunity, ease of adoptabil­

ity to existing installations, arc flash concerns, etc. Present groundinglbonding practices in telecom/datacom installations in Europe, NA and Japan are governed by slightly differ­ent concepts, practices, codes or regulations. Many different grounding configurations are possible for 400VDC distribution [4].

This paper addresses only those which are practical and acceptable for uniform, global implementation.

II. OVERVIEW OF FRAMEWORK (EQUIPMENT AND

DISTRIBUTION ENCLOSURES) GROUNDING CONCEPTS

A. Reasons for grounding and bonding of electrical equipment

Experience in the operation of telecom/datacom electrical distribution systems provides the following basic reasons func­tions and attributes for a grounding (bonding network) [5],

Ensuring personnel safety is the most important function of grounding system. Europe, North America (NA) and Japan have their own guideline documents addressing grounding today (see. TABLE I). Although main concept of protection against fire-hazard and ensuring human safety is the same in all the regions, it's precise implementation, such as wiring concept and wire sizes are slightly different.

1) Provides personnel safety and reduces fire hazards -mandatory requirement

2) Enables signalling with earth return; 3) Minimizes service interruptions and equipment damage; 4) Minimizes radiated and conducted electromagnetic

emissions; 5) Reduces radiated and conducted electromagnetic suscep­

tibility; 6) Improves system tolerance to discharge of electrostatic

energy, and lightning interference.

Other practical requirements include:

1) Ease of implementation and verification; 2) Flexibility in application in existing facilities; 3) Cost.

B. Grounding topologies in Europe, NA, Japan

Figure 2 shows the basic grounding (bonding) topologies in use today. Two main methods can be classified as "Star topology" and "Mesh topology". Star topology which is called Star-IBN (isolated bonding network) is commonly used for switching facilities in Japanese telecom buildings. Star IBN bonding (grounding) network has one Main Earth Terminal (MET, in US referred to as SPCB-Single Point Connection Busbar or MGB - Main Ground Bus). MET serves as a single point connection (SPC) to ground for a bonding network. Multiple bonding networks can be connected to a single MET [5].

The advantage of star topology is ease of locating the ground fault current flow, and easy connectivity to another grounding network. The disadvantage is difficulty in main­taining the grounding network isolation from the building

TABLE I BASIC STANDARD OF SAFETY AND GROUNDING

Area Basic mandatory safety requirement

Europe - Cenelec EN 60950 (Based on IEC 60950)

US - National Electrical Code

Japan - Electric Utility I ndustry Law

- JIS-C 6950 (Based on IEC 60950)

Recommendation of bonding networks in telecom/datacom building - ITU-T K.27 - ETS 300 253 - Telcordia GR-295-Core - Telcordia GR-I275-Core - Individual carrier standards - TIA-942 - J-STD-607-A-2002 - IEEE Std 1100 (IEEE Emerald Book)

- Electrical equipment technical standards

- ITU-T K.27 - JIS-C60364

(Based on IEC60364) - JEITA IT-1004 - JEITA ITR-1005

S8a���J888l� --spew spew Mesh-BN Mesh-IBN Star-IBN

Fig. 2. Most common grounding topologies.

structure in large facilities (proper installation and maintenance practices must be strictly followed).

There are two types of Mesh topology - Mesh-IBN which is isolated from the building conductive structure and Mesh-BN which is connected as multi-point grounding plane. The merit of Mesh topology is that due to low ground impedance it is easy to obtain equal potential between all frameworks in elec­trical system. Demerit is difficulty in locating objectionable ground fault current paths. Mesh-BN facilitates construction of a new grounding network (for example addition of a new equipment rack) but noise current from another ground network can propagate through the building structure.

The view on framework grounding topology has changed in historical perspective. In Europe, at one time, it was important to separate earth in the buildings from clean earth of sensitive electronic equipment. Different star topologies were widely applied to isolate grounding problems. With more electronic equipment introduced in the buildings, the grounding com­plexity has significantly increased. Transients and surges have become more difficult to isolate in star topologies due to unin­tended ground loops introduced during equipment installation and inconsistent wiring practices. Uncontrolled transients in the poorly grounded network can easily cause resonance and instability.

Lightning problems, increased equipment complexity on sites and increased requirements for EMC performance have contributed to switching to more multi-point grounding so­lutions of Mesh type. Multi-point grounding results in lower impedance grounding that can better protect people and sensi­tive equipment from transients caused by equipment failures,

lightning strikes or other system issues. The above issues contributed to selection of Mesh-BN as a preferable framework grounding topology in Europe. In NA and Japan the grounding topology preference is dependent on a specific application and in US may differ between individual carriers. In Japan, in telecom network switching facilities Star-IBN is a standard, while radio equipment is using Mesh BN concept.

There is no ideal grounding topology , selection of the grounding method is often based on company preferences and objectives. No matter which topology is selected ,the proper installation practices are a fundamental element of a well functioning grounding system.

Since all three framework grounding topologies are in use today, the recommended high resistance midpoint grounding of 400VDC distribution systems must be easily adoptable to all three concepts.

III. DC AND AC RETURN TOPOLOGIES

A. General (framework or protective earthing vs grounded

return)

Framework (equipment) grounding (also called earthing) topologies should not be confused with different methods of applying DC return path with reference to earth and framework grounding. Framework grounding and DC return are two different concerns and although related to each other require different considerations. Framework grounding is con­trolled by the mandadtory, official regulations while DC return grounding methods are praxis adopted by the industry or individual carriers.

This section explains DC return methods used in -48VDC systems today. Relation between AC return and framework grounding is also briefly discussed in this, since majority of the data centers today utilize AC distribution.

B. 2- and 3-wire DC return

In 2-wire concept, DC return in each cabinet is connected to the cabinet chassis, see Fig. 3 and Fig. 4.

In 3-wire DC return concept DC return through OV lead is separated and isolated from framework grounding except for a single connection point in the main power cabinet, see Fig. 5.

Both 2-wire and 3-wire concepts are implemented in the field today and both have their advantages and disadvantages.

In a correctly dimensioned 2-wire power system all DC returns shall be through OV leads and no currents should flow through framework grounding structure.

All the frameworks are a part of a large common ground­ing reference plane. The advantages of 2-wire concept are: improved EMC performance due to enhanced multipoint grounding plane and improved safety performance during short circuits (since it doesn't matter if short circuit in Equipment Cabinet occurs between -48V and chassis or between input terminals of the equipment). The OV lead will carry full DC return current (no current flows in the framework structure, if correctly dimensioned) in both short circuit cases. The weakness of a 2-wire return system is a risk of partial DC return current flow through framework structure. This occurs

DC UPS Equipment

48v.---+-------------�--e-48V

OV �L�-------+[J--. .

····· -············�·····2·�it��-��ti��� •• I

OV

MET (Main Earth Terminal)

Fig. 3. 2-wire concept in EMEA and North America (DC return through OV lead & framework).

DC UPS Equipment

48V 48V

OV L U OV

• I MET (Main Earth Terminal)

Fig. 4. 2-wire concept in Japan (DC return through OV lead).

DC UPS Equipment

48V.---+-------------�� 48V

OV �L-+--------�-- OV

···················,..·····2·�ii����ii��� 1 �

• • l MET (Main Earth Terminal)

Fig. 5. 3-wire concept (DC return path through OV lead).

when impedance of DC return - OV lead is not low enough as compared to impedance of DC return framework.

The advantages of 3-wire concept is elimination of DC re­turn current path through framework structure (due to isolation of OV lead). The main disadvantage is a risk of a short circuit from -48V to chassis in the Equipment Cabinet causing a large return current (can be several thousands A depending on the battery size) to flow back to the battery entirely through the framework structure. It is extremely important to correctly dimension all protection cables for framework. In addition, multipoint grounding structure is less effective in improving EMC performance in 3 wire concept.

C. AC return in AC network

AC return is always separated from framework grounding in data centers. AC return path is either through Neutral (N), as in Europe and 120V systems in US or through one of the

phases of 208VAC, as in US and Canada. AC return through N in Fig. 6 corresponds to one pole grounded 48V system and 3-wire concept in Fig. 5. In both cases the return is connected to Earth at one point and otherwise isolated from framework grounding.

Typical supply voltage for ICT equipment in US, Canada is 208VAC ph-ph. Each phase is symmetrical to Earth. The AC network lay-out in Fig. 7 corresponds to direct midpoint grounded DC system in Fig. 8. In both cases power supply lines are symmetrical to Earth and short circuit of one pole to ground activates protective circuit breaker/fuse due to low ohmic path to Earth. Both supply lines L I and L2 have to be protected by protective devices.

Framework or equipment grounding is considered to be a basic safety requirement and is mandated by the standards (listed in TABLE I). The size of equipment grounding con­ductor is specified in these documents and is related to the size of the upstream overcurrent protection device (example NEC Table 250.122). For US this table applies to both AC and DC systems.

D. Midpoint grounding in DC network

In principle many grounding methods such as one pole, direct midpoint, high resistance one pole, high resistance midpoint can be applied on 400VDC facility distribution. In addition most of the DC UPS/converters configurations can accept any of the existing AC grounding systems (TN, TT, or IT). ICT equipment can also accept all 400VDC ground­ing methods. However, high resistance midpoint grounding

TN Distribution : Network :

L3 e------:...-L2 L1 N

PE

e------:...-

_L

ACUPS L !.-:-I

N

PE I I I

Equipment

L1 230 Vac

N PE

T I T

•• I = MET (Main Earth Terminal)

Fig. 6. AC return through N in AC network, Europe.

TN Distribution : Network :

L3� L2

L1 N PE

:

_L

ACUPS L2.-:-t

L1.-:-I PE 1 T I

Equipment

L2 208 Vac

L1 PE

T I I

• • I = MET (Main Earth Terminal)

Fig. 7. AC return through a phase line, NA.

becomes a favorable method when putting safety aspects as number one priority.

Midpoint grounding in DC network is considered only for higher DC voltage levels. For 24VDC and 48VDC the simplicity of one pole grounding is a decisive selection factor. Two basic midpoint grounding concepts can be considered in DC distribution network.

1) Direct midpoint grounding (Fig. 8) 2) High resistance midpoint grounding (Fig. 9)

Direct midpoint grounding method in Fig. 8 is not com­monly utilized in DC networks. The main disadvantage is a risk of loosing midpoint grounding connection, if the battery for some reason is disconnected or not used at all.

High resistance midpoint grounding as shown in Fig. 9 is widely used in 110VDC and 220VDC networks in industrial, utility and railway environments today. It brings enhanced personal safety (low human body current) and eliminates the risk of arc flash at short circuit to ground.

High resistance grounding shown in Fig. 9 has been suc­cessfully tested in France Telecom and NTT 400VDC pilot trials.

IV. PERSONNEL SAFETY CONSIDERATIONS FOR 400VDC DISTRIBUTION SYSTEMS GROUNDING CONFIGURATION

A. TT d.c. system

As explained in previous chapters, the safe operation of the higher voltage direct current distribution is of major importance.

I

DC UPS Equipment

+200 Vdc +200 Vdc -200 Vdc -200 Vdc

(optional)

- - n r --------------- -------- ---------Battery

I I-< H I I I • •

MET (Main Earth Terminal)

Fig. 8. Direct midpoint grounding in 400VDC network.

DC UPS Equipment

+200 Vdc +200 Vdc

'I.;.Vdc -200 Vdc (optional) _on PE r - - - - - - - - - - -- - - - - - . _ - - - - - - -- - -- - -Batterv

IY�� I I 1 I • • •

= MET (Main Earth Terminal)

Fig. 9. High resistance midpoint grounding in 400VDC network.

It is desirable to reduce the touch voltage personnel can be exposed to a value similar or lower than 240VAC. This can be accomplished by directly grounding mid point of the output of 400VDC power supply (Fig. 8) to limit the human body exposure voltage from either polarity to ground to 200VDC (1/2 line voltage). IEC60364 [6] classifies such a system as TT d.c. (Fig. 10) .

The effect of the electric shock on human body also depends on the voltage polarity reference [7] and the current path through the body.

The causes of death by electric shock are divided into ventricular fibrillation, bum injury, and secondary disasters such as falling from heights caused by electrocution. Among these causes, ventricular fibrillation has the highest fatality rate. Therefore, we review ventricular fibrillation from elec­tric shocks in a 400VDC system in more detail. Ventricular fibrillation is a phenomenon caused by current flow through the heart. It brings on a convulsion of the cardiac ventricle, disturbs pulse, and circulation of blood is stopped, which may lead to death.

Physiological phenomena resulting from ventricular fibril­lation by electric shock have been studied. Studies by Dalziel (USA) and Koeppen (Germany), in particular, are well known [8], [9]. They collected and analyzed a large amount of data on ventricular fibrillation from human and animal tests. These data has become the foundation of knowledge on electric shock.

The magnitude of ventricular fibrillation is determined by the following parameters.

I) magnitude of current flow through the heart 2) duration of current flow 3) current path 4) type of voltage (AC or DC)

In particular, the magnitude of current flow through the heart is an important parameter that determines the occurrence of ventricular fibrillation. This current flow is related to the electrical impedance of the human body.

Figure I I shows a simplified characteristic curve of body current/duration of current flow for the hand-to-feet pathway [10]. The electric shock level is classified into the area sand­wiched between the lines of AC or DC. The roman numerals in this figure express the level of electric shock.

I Normally no perception II Normally no dangerous effect with little pain III Normally reversible effects such as muscular con­

traction and respiratory compromise IV Critical effects, such as ventricular fibrillation

According to Fig. I I , the harmless DC current is larger than that of AC. It is generally said that DC electric shocks are less dangerous than AC ones. Ventricular fibrillation is largely affected by the direction of current through the heart. The heart-current factor F is considered as only a rough estimation of the relative danger of the various current paths regarding ventricular fibrillation.

F is expressed as

Source Installation

�----r------o--------+-- L+ :-:-: --.. -:--,-- - -0:------1--..-- M '" --� I I " :-:-: ----} -+ - .-o-+----+--<.-t-- L-

I I I I I I

(jj' 5 3: 0 c;::: E Q) t: ::::J t.) '0 <:: ,g � ::::J

0

I I I PE Optional -r ' I I - .,

---. I I 1 I application of .,...- � I [- -- - - - - : a battery _'-' : � =- E-xP�;�d:;;"-d-u-�tive-part

-----" Earthing

10000

1000

100 1-1

of system Earthing of exposed -conductive-parts

Fig. 10. TT d.c system.

AC

� DC --

!I III I�\

� I 10 100

Body current (rnA)

d

-

1000 10000

Fig. 11. Characteristic curve of body current/duration of current flow, IEC/TR 60479-5.

F = Ire! h ' (1)

where F: heart-current factor, Ire!: body current for left-hand-to-feet path, h: body current for each path.

Table II lists the heart-current factors for different current paths. It was found that F is large when body current passes around the heart. When the current passes through the heart horizontally, F becomes about a half and ventricular fibrilla­tion usually will not occur. Voltage of DC has a polar character (positive and negative electrodes). Therefore, if one receives a shock in a DC system, the current direction through the body is constant. It is understood that the threshold of fibrillation for a downward current is about twice as high as for an upward current.

A midpoint grounding system could suppress the voltage to ground by half of that in a one-end grounding system. However, in this system, the effects of electric shock are different if one receives a shock on the positive line or negative line.

When one receives a shock on the positive line (X in Fig. 12), the touch voltage is 200 VDC with human resistance of 1,500 n, and the body current is 147 rnA. In this case, the breaking limitation time is 180 ms. To protect the personnel the

TABLE II HEART-CURRNET FACTOR F FOR DIFFERENT CURRENT PATHS

Current path Left hand to left foot, right foot or both feet

Both hands to both feet Left hand to right hand

Right hand to left foot, right foot or both feet Chest to left hand

Left foot to right foot

Heart-currentfactor F 1.0 1.0 0.4 0.8 1.5

0.04

circuit must be equipped with an RCD (residual-current device in NA referred to as GFCI ground fault circuit interrupter) with sufficiently short interrupting time. For 147 rnA current there is an RCDs (compliant to JIS standards that can protect the human body).

On the other hand, when shock occurs on the negative line (Y in Fig. 12), the body current flows upward through the heart, and F doubles, and the current which can cause ventricular fibrillation is 147 x 2 = 297 rnA. This current value is almost the same as in case of a directly negative grounding system. From the safety prospective, TT d.c. system is almost as dangerous as a direct negative grounding system. Based on the above this system does not adequately address the personnel safety issue.

B. High resistance midpoint grounding as IT dc. system

IEC60364 classifies a system as IT d.c. system (Fig. 13) when one polarity of the power source is grounded by a sufficiently high impedance and the battery is isolated from the framework.

The high resistance midpoint grounding in Fig. 9 is a derivative and an attractive variant of IT d.c. The touch voltage will be reduced and the amount of current flow through the human body at ground fault can be kept at safe level preventing electrical shock as well as fire hazard to the equipment, see Fig. 14.

Body current can easy be limited to safe region II, 2-25 rnA (Fig. 11) in continuous operation when the value of the midpoint resistance is correctly selected.(Fig. 9 and 14). The resistance value in 220VDC industrial/utility high resis­tance midpoint grounding applications is normally 50 kD or below, limiting the body current at ground fault to 2-5 rnA range. No RCDs are required for earth current detection and disconnection since current flow is limited to harmless level.

Comparison to 230VAC L-N systems indicates that high resistance midpoint DC grounding brings significantly higher safety levels. In 230VAC case the body current is in 150 rnA range (230/1500) that results in dangerous zone IV (AC­characteristics), Fig. 11. The use of RCD is a mandatory requirement in 230VAC L-N systems in Europe to prevent a potentially lethal electrical shock.

C. Need of ground fault monitoring in high resistance mid­

point grounding topology

The high resistance midpoint grounding limits short circuit currents in case of a first incidental ground and allows contin­uous equipment operation. However if one pole is incidentally

x +200 V de

-200 V de � L.....-___ ----' � Y

Fig. 12. Circuit schematic of direct midpoint grounding system.

Source Installation

�----f------ L + iLd----t--,---9-----t-.--- L-

: : 1l PE

Optional

I rL ai I I I � I I I a.. : !...: � I C)

application of -r' I I I a battery I -=-

Earthing of system

i- - - -- I . . :- �;PO-S�d��ductive-part

Earthing of exposed -conductive-parts

Fig. 13. IT d.c system.

+200 V de R - _ .. _ - - . .

-200 V de, R

- . I .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..

Fig. 14. Current path through human body in high resistance midpoint grounding.

grounded ,the voltage between the ungrounded pole and earth will be elevated to 380VDC. Second short circuit on the opposite pole will cause protective device to clear the fault. It is important to detect and eliminate the first ground fault in reasonable time to minimize the risk of double ground fault and avoid personnel exposure to 380VDC.

High resistance midpoint grounded system requires imme­diate investigation and clearing of ground fault even though the ground fault current is in rnA range. As long as pole­to-ground fault does not escalate into additional pole-to­ground on opposite polarity resulting in pole-to-pole fault and operating the protective device as circuit breaker or fuse, the continuous operation can continue. However, it is essential to monitor and alarm on the first pole-to-ground fault.

Fig. 15 shows the monitoring principle in high resistance midpoint grounding. The insulation detector unit that is inte­grated in the DC UPS generates midpoint grounding on DC bus and monitors the current difference for each load branch. 6.1 is sensed when the leakage to ground occurs at the branch.

D. High resistance midpoint grounding in Star-IBN, Mesh-BN

and Mesh-IBN

High resistance midpoint grounding can successfully be implemented in all three framework grounding topologies Star­IBN, Mesh-BN and Mesh-IBN. The main requirement for all three applications is to ensure safe connection between MET and the midpoint of two grounding resistors, see Fig. 9 and 15.

V. EMI CONSIDERATIONS

Suppression of EMI is an important function of the effective grounding system. Results of the study conducted by NTT [11] indicate that the conductive noise voltages of mid-point resistive grounded 3S0VDC system were not notably different from a directly, one pole grounded system. In adittion there are no reported EM! related issues or problems from any of the existing proof of concept sites and commercial deployments.

VI. CONCLUSIONS

One pole direct grounding is a simple and well estab­lished method for low DC voltage networks 24 V and -4Sy' High resistance midpoint grounding is widely used today in 110VDC and 220VDC networks in industrial, utility and railway environments. Practical experience from the 220VDC deployments suggests the validity of the same topology for 400VDC distribution systems.

The mid point resistive grounding system provides better personnel safety than other alternatives ,both 20S/240VAC (or higher voltages AC) and other grounding methods for higher voltage DC. One pole grounding in 400VDC (either TT d.c. or IT d.c.) is not an acceptable method from personnel safety and arch flash risk point of view. Direct midpoint grounding is an improvement in that respect, but can still produce excessive currents in human body (polarity dependant) for maintenance personnel. This topology dependence on the battery connection and its continuous presence limits the application field for 400VDC UPS.

High resistance midpoint grounding limits ground fault current in human body to harmless levels ensuring personnel safety and eliminating arc flash risk during single ground faults. It can be easily implemented in the existing framework grounding topologies Star-IBN, Mesh-BN and Mesh-IBN. In addition it permits distribution system operation with single ground faults. The disadvantage of high resistance midpoint grounding is a need for ground fault current monitoring. This topology is well understood and has been in use in industrial and other applications.

Based on careful examination of merits of different ground­ing methods high resistance midpoint grounding has been

DC UPS system

+200 Vdc

,- -, Leakage : .... : resistance

Fig. 15. Ground fault monitoring in high resistance midpoint grounded 400VDC system.

selected as a preferable grounding topology for 400VDC dis­tribution systems by NTT, NTT FACILITIES, France Telecom, and Emerson Network Power and is highly recommended for other users.

ACKNOWLEDG MENT

The authors would like to thank Didier Marquet, Stephane Le Masson (France Telecom) and Kaoru Asakura (NTT) for their guidance and contribution to this paper.

REFERENCES

[I] Peter Fairley, "Update; Edison Vindicated; Efficiency tempts computer centers to go DC," IEEE spectrum, pp. 14-15, Feb. 2011.

[2] IBM Press release, "IBM, Syracuse University, New York State to Build One of the World's Most Energy-Efficient Data Centers," http://www-03.ibm.com/press/us/en/pressrelease/27612. wss, May 2009.

[3] T. Babasaki, T. Tanaka, K. Asakura, Y. Nozaki, K. Fujio, "Development of power distribution cabinet for higher-voltage direct-current power feeding system," Proc. of the IEEE Intelec 2010.

[4] White paper No. 3 I, Issues relating to the adoption of higher voltage direct current power in the data center , The Green Grid, 2010.

[5] ITU-T Recommendation K.27, Bonding configurations and earthing inside a teecommunication building, May 1996.

[6] IEC 60364-1 ed5.0, Low-voltage electrical installations - Part 1: Fun­damental principles, assessment of general characteristics, definitions, 2005-11-29.

[7] M. Noritake, T. lino, A. Fukui, K. Hirose, M. Yamasaki,"A study of the safety of the DC 400V distribution System," Proc. of the IEEE Int elec 2009.

[8] F. Dalziel, "Reevaluation of Lethal Electric Current," IEEE Trans. Industry and General Applications, IGA-4 Vol. 5, pp. 467-476, 1968.

[9] S. Koeppen, et ai, Elektrounfalle und iher Einflussgroben aus medizinis­cher Sicht, ETZ-B H. 6, 1966.

[10] IECITS 60479-1 ed4.0, Effects of current on human beings and livestock - Part I: General aspects, 2005-07-13.

[II] T. Tanaka, Y. Honma, S. Kuramoto, T. Tanaka, T. Babasaki, Y. Nozaki, "Basic study on grounding system for high-voltage direct current power supply system," Proc. of the IEEE Intelec 2010.

ApPENDIX

GLOSSARY

CBN: common bonding network

EMC: electromagnetic compatibility

EM!: electromagnetic interference

EN: European standard

ETS: European telecommunication standard

IBN: isolated bonding network

ICT(E): information and communication technology (equipment)

IEC: the international electrotechnical commission

ITU-T:the international telecommunication union standardization sector

IEITA:the Japan electronics and information technology industries association

JIS: Japanese industrial standards

MET: main earthing terminal. In US referred to as single point onnection busbar (SPCB) or main ground bar (MGB)

PE: protective earthing

RCD: residual current device (in US and Canada also known as GFI or GFCI)

SELV: safety extra low voltage (in IEC, less than 25 V ac or 60 V dc)

SPCW: single point connection window

TIA: telecommunications industry association

UPS: uninterruptible power supply