In telecom system grounding is positive while at industrial and other electrical grounding is negative and yet they were tied up together . Why?

First of all, we need to see the difference between telecommunication load and common electrical load. In common electrical load which is dominated by rotational machine, an AC line,1 phase or multiphase is commonly used to distribute the power because it has significant advantages especially in power distribution efficiency, simplicity of connection, and cost of distribution line (smaller cable). In this AC system, we have a few type of grounding connection like T-T, T-N, T-NC, IT etc. To be noted, this grounding system doesn't specify positive or negative grounding because the system is AC with no specific positive or negative poles.

IMHO, Telecommunication system application, which is now is blurred with IT system, conventionally use a negative grounding system. Usually the system is -48VDC (or -24VDC in some special old cases) which is grounded at positive pole. While for digital electronics system which were the basics of IT equipment, conventionally use +5VDC for TTL ICs or3.3VDC for CMOS type ICs. Sometimes they also used12VDC for mechanical interface. But since the advanced development of SMPS (switching mode power supply), we can easily convert among these power system.  

The factors why we use this negative DC system are:

- DC was used to carry voice signal in conventional telecommunication. For comparison, in Laplace or Fourier analysis the s variable for pure DC system is zero. So it is easier to sort the signal and noise function.

- AC Power was not deemed reliable enough for critical telecom application.

- Telecommunication and IT application Quality of Service mainly depends on service availability. So they need a stored energy backup (UPS) which is usually developed on DC system. For comparison, AC system cannot be easily stored and transfer switching system usually disturb the signal quality by the transient current and switching characteristic.

- Negative polarity reduces electrolytic corrosion problems with underground cables and conduits which were lead sheathed.

- Common available batteries usually12VDC which can be multiplied to24V or48V.

- ELV (<72V or60V) did not require licensed electricians.

- -48VDC was not governed by the NEC.

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