What does the 6,000 amp short-circuit current mean for the DC protection?

It is the figure the fuse or breaker on each string has to be able to interrupt. The sheet puts the NSB190FT HT at about 2.3 milliohms and 6,000 amps, and because voltage and resistance both add along a series string, a -48 volt string of four delivers roughly the fault current of one block. Parallel strings behave differently: their fault currents add, so four strings on a common bus can present up to about 24,000 amps before cable resistance is counted. Size the interrupting rating of the string protection against that total, and send the string count with your quote request so the specialist can check it.

What is the difference between HT RED and standard RED?

The warranty and the temperature behavior. Capacity is unchanged between them: both are thin plate pure lead front-terminal blocks with the same 15-year design life at 20°C, and both operate from -40°C to +65°C. The HT RED build adds sustained operation at 40°C (104°F) as a published characteristic, and EnerSys warrants it for four years where standard RED carries three. If the cabinet runs hot, the HT build is the one that holds up on the site.

How do I size a string on 190 Ah?

Work from the reserve time and the load, not from the amp-hour label. Telecom reserve is normally specified as a load held for a number of hours to a defined end voltage, and the capacity available falls as the rate rises. Divide the plant load by the number of parallel strings to get the current each string must supply, then check that current against the reserve duration you need. A specialist will run the calculation from the plant load, the reserve target and the end voltage if you send them, and will state the string count in writing with the quote.

Can I add one new block to an aged NSB190FT string?

Rarely a good idea. In a series string the weakest block sets the runtime, so a new block put into an aged string ages down to the string rather than lifting it, and the replacement delivers less than its own sheet says. If the string is young, a single early failure is worth a conversation before anything is bought. If it is several years old, the honest answer is usually to plan the whole string and keep the good survivors as spares. Send the install date and the last set of float readings and a specialist will tell you which case you are in before anything is ordered.