Desktop UPS versus Centralised Systems

Desk Top UPS ( Uninterruptible Power Supply ) versus Centralised Systems

A small desktop UPS is a typical line interactive unit. It is intended for the man on the street to use at his home, not to run data centres. This means that the UPS is essentially off all the time and will switch on when the mains fails, so it has a switching time i.e. a dead or off time between mains failure and switch on. It relies on the capacitors in the PC power supplies to bridge this period.

As the unit is not double conversion, it does not protect the PC from spikes, dips, brownouts etc. that occur on the mains supply grid as an online double conversion unit will. Remember that more than 80% of IT failures and software corruption is as a result of bad mains, not mains failures.

In the marketing of a unit like this the manufacturers always boast boost and buck abilities, filtering etc. but at such a low purchase cost it is impossible to produce equipment that does this properly. For instance, you can protect for lightning / surges by using a MOV (Metal Oxide Varistor) and buy a false sense of security (Costs almost nothing) or if you want to do it properly and effectively by using the proper equipment that is 1000 times the cost of a MOV but will protect your load properly. Both are so-called surge protection devices but one will do the job properly, the other will only survive very minor surges.

If you want to protect a PC properly then there is no alternative but to use an online double conversion system. The problem is that some of these units are going to induce an incredible amount of harmonics back into the supply transformers and generator which will cause a different set of problems.

Decentralised systems have been used in the past and always failed. In the early ’90s there was a push in the industry to move to small units. It never worked properly for many reasons and the industry then moved back to centralised systems.

These units have one advantage i.e. low initial cost; however, the disadvantages are numerous:

• There battery chargers are not ideal with high ripple so batteries last only about 2 years.
• The batteries are internal so the high heat also affects their life expectancy.
• The batteries have to be replaced every two to three years resulting in excessive replacement costs negating the initial low capital expense.
• These units are actually intended to be “consumable” products and replaced when the batteries have been depleted, however, manufacturers cannot confirm this due to potential environmental backlash.
• Particular mention should be made of potential fire risks, especially so in the case of older batteries in the units. Should the MOV blow, the damage to the environment also dramatically increases.
• Due to fire hazard, it is advisable to inspect each unit bi-annually to ensure that the batteries are not leaking or swelling. These inspections visits further add to the units running costs.
• Even on generator these units will occasionally switch to battery and beep, e.g. when the AC pumps etc. kick in and out and cause the supply voltage to vary. This is due to the design of the product.
• Support on these units is almost non-existent; if they break you throw them away and buy a new one as it is too costly to fix them.

The list is endless.