SIM100MLP datasheet(4/19 Pages) SENSATA

4 / 19 page

Preliminary

Rev 1.1a

Sendyne SIM100MLP

Safety of the IT power system

Chassis

Figure 1: The IT power system topology

Figure 3: In a “symmetrical” isolation fault V

n = Vp

Chassis

Figure 2: Single isolation fault

Ungrounded, unearthed, “floating” or IT (Isolé terr

or Isolated Terra) are all terms used to describe power

systems that have no intentional conductive connec-

tion to earth’s or chassis ground. The main advantage

of the IT power system is that a single “short” will not

disable its ability to continue delivering power. Figure

1 illustrates the basic topology of such a system.

The resistive connections, shown in Fig 1, between

the terminals of the power source and the chassis are

referred to as the “isolation resistances” ( R

ISO,P and

ISO,N ) and they represent the parallel combination of

all resistive paths from the power source terminals to

the chassis (including the ones the isolation monitor

introduces). The values of isolation resistances are

desirable to be high so leakage currents that travel

through them are kept to a harmless minimum. The

capacitors shown represent the parallel combination

of all capacitances present, including the Y-capacitors

typically used in DC IT systems to suppress EMI. The

values of Y-capacitors are kept within limits in order

to avoid hazardous accumulation of energy. The volt-

ages V

p and Vn are shown each to be equal with half

the battery voltage, which will be the case if the values

of R

ISO,P and RISO,N are equal.

Isolation faults

If either of the isolation resistances decreases below

the threshold of 100 Ohms/Volt a hazard occurs if a

person makes contact with the terminal “opposite”

to the leaking resistor. This hazardous situation is il-

lustrated in Figure 2.

Chassis

Figure 4:

An isolation fault may originate from any

point within a battery pack

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