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TESTING OF INFORMATION TECHNOLOGY EQUIPMENT

Information technology equipment such as personal computers has a tendency to leak current to earth as an operational characteristic.

These currents are the result of intentional and unintentional capacitive links with earth.

Although the insulation resistance may be very high this will have no effect on capacitive leakage, which cannot be detected by insulation resistance testing.

The mains input to a computer will contain transient voltages at varying frequencies, which will corrupt data if allowed to enter the circuitry. Filters will usually be installed in the form of capacitors that will remove these high frequency currents by earth discharge.

A capacitor will have high impedance to low frequency supplies but as the frequency raises the impedance of a capacitor falls and hence provides a leakage path for high frequency current only. As stated previously, the impedance of a 1 mF capacitor at 50 Hz is 3 183 ohms and at 50 kHz, 3.18 ohms.

If these leakage currents are excessive they may trip an RCD and the cause could be ambiguous to the investigating electrician, as insulation resistance values may be high. A measurement of current by a standard clamp meter could possibly reveal no leakage current due to a restricted response to high frequencies.

B.S. 7671 makes the following requirement regarding permitted earth leakage current.

Regulation: 607-02-03 Where more than one item of equipment leaks more than 3.5 mA to earth, the total earth leakage current shall trip any protecting RCD

Regulation 607-02-07 requires that where the total earth leakage current exceeds 10 mA the protective conductor will be duplicated or be of a minimum c.s.a. of 10 mm.² or be protected by an earth monitoring device.

The previously described clamp meters may measure such a current.

I

Fig.55

Inspection and Testing. A.W.Croucher 2 004 page 88

As with all instruments operated by electromagnetic induction, all live conductors (P - N) will have to be embraced and the residual current may then be measured. If the connecting flexible cord is a 3 core then an adapter will have to be constructed which will enable the exclusion of the protective conductor.

three core flex adapted for current measurement.

Regulation 607-02-03

Where more than one computer is connected to a circuit the total earth leakage current is not to exceed 25% of I∆n

Ι ∆n >= 3.5 mA

Fig.56

Fig.57

Inspection and Testing. A.W.Croucher 2 004 page 89

A single protective conductor shall have a minimum c.s.a. of 10mm.2

Where protective conductors are duplicated they shall have a minimum c.s.a. of 4.0mm.2

Regulation 607-02-07

If a single item of equipment has an earth leakage current in excess of 10mA, one or more of the following arrangements must be made.

A duplicate protective conductor within a multicore cable. Total cross sectional area is to be not less than 10mm.2

Fig.58

A single protective conductor within steel trunking or conduit.

Minimumc.s.a. 4.0 mm.2

Inspection and Testing. A.W.Croucher 2 004 page 90

Regulation 607-02-04 I

Earth leakage current exceeding 3.5mA but not exceeding 10 mA

connection by means of an isolator

connection by means of a BS 4343 plug and socket

Regulation 607 - 02 - 05 Earth leakage current exceeding 10 mA

BS 4343

E

BS 4343 plug

4 core flex E

Two protective conductors required - the supplementary earth must be supplied via a separate contact and have a c.s.a. of 4.0 mm.2 or greater.

Regulation 607 - 02 - 06 where the total earth leakage current exceeds 10mA the circuits shall be wire in a ring with no spurs

Fig.59

Fig.60

Fig.61

Inspection and Testing. A.W.Croucher 2 004 page 91

Regulation 607 - 02 - 06 where the total earth leakage current exceeds 10mA the circuits shall be wire in a ring with no spurs

Fig.62

Inspection and Testing. A.W.Croucher 2 004 page 92

SECTION 19

It has long been the requirement of wiring regulations that the prospective short circuit current should not exceed the safe disconnecting capability of the controlling fuse or circuit breaker. The definition of prospective short circuit current is,

"That current which will flow between solidly bolted conductors of differing potential. In the case of a three phase and neutral installation, the conductors in question will be all three phases."

Obviously, the prospective short circuit current of an installation cannot be determined by the above method; measurements will be taken using a dedicated prospective short circuit current tester. This instrument, when connected to the installation, will circulate a significant test current that will cause a small mains voltage drop. This fall in voltage and the current that produced it will be integrated by the instrument, which will then calculate the mains impedance and the potential resulting short circuit current.

Prospective short circuit current testers are usually designed to be attached to only two points on the installation and therefore cannot be directly used to determine current resulting from a symmetrical three phase fault.

Theoretically it can be shown that a symmetrical three-phase fault will produce approximately twice the current produced by a single-phase fault; if a single-phase measurement of PSCC is made, the recorded value should be doubled. When using an instrument that connects to two lines (400V) the correction factor is x 1.15

In reality, joint and cable resistance and resistance changes produced by the inevitable temperature rise that results from current flow considerably attenuate short circuit current. The actual three-phase fault level will be somewhat lower than that calculated, however any error would be on the safe side.

When assessing the suitability of an overcurrent device, having determined the apparent fault level, caution should be exercised. Upstream devices such as HBC fuses, miniature and moulded case circuit breakers may be "current limiting". This term means that the device will

"cut off" before the prospective current is reached. For example, the prospective short circuit current at the bus bars of a distribution board may be assessed at 25 kA, but the upstream overcurrent device may be designed to limit the current to 9 kA hence allowing the installation of cheaper downstream circuit breakers or fuses. It will have to be ascertained of course by the designer that discrimination has been achieved for all fault levels up to 9kA, beyond which it will be lost.

Short circuit breaking capacity of differing overcurrent devices are indicated in the tables overleaf.

Fig. 63

MEASUREMENT OF PROSPECTIVE FAULT CIRCUIT CURRENT Reg. 434-02-01

Inspection and Testing. A.W.Croucher 2 004 page 93

It should be noted that if a prospective fault level of 20kA were to be measured at the origin of an installation, a fault only one metre from that point, connected by a copper cable of 10 mm.2, would attenuate the current to 17.4 kA.

Short circuit breaking capacities for differing protective devices.

Types of device Breaking capacity (kA)

Fuses to BS 1361 type 1 16.5

A symmetrical three-phase fault

Table 9

Inspection and Testing. A.W.Croucher 2 004 page 94

Fuses to BS 1361 type 2 33.0

Fuses to BS 88 80

Fuses to 1362 6.0

Fuses to BS 3036 according to category of duty marked on the fuse link, as follows.

Miniature circuit breakers to BS 3871 and BS EN 60898

S1 1.0

S2 2.0

S4 4.0 for current ratings

from 30A up to and including 100A

M1 1.0

M1.5 1.5

M2 2.0

M3 3.0

M4 4.0

M6 6.0

M9 9.0

Circuit breakers to BS 4752 - As per marked breaking capacity.

Inspection and Testing. A.W.Croucher 2 004 page 95

SECTION 20