Craig and Derricott — Switch Doctor

Your questions diagnosed & answered by Switch Doctor. If you have a question please send it using the form on this page. We will maintain your anonymity in any answers unless you indicate otherwise.

“I have to isolate a motor with a 90 kilowatt power rating and a maximum current of 125 A but I am told I need a 250 Amp isolator – why?”

Isolators or Switch Disconnectors have to be rated to a European Standard BS EN 60947-3 which rates the Isolator based on kilowatt power rather than current.

This means that power factor losses are taken into account and a properly rated Isolator will be able to break the stalled motor current in an emergency situation.

“I need to mount some isolators on a roof of a building near the sea and my consultant has recommended stainless steel enclosures. Why can’t I use sheet steel, after all this has IP65 protection?”

The IP rating only relates to the ability of the enclosure to withstand dust and water, it does not guarantee that the finish of the isolator enclosures will not deteriorate over time.

If you think of a car, this is sealed against dust and water but if you don’t maintain the bodywork it will eventually go rusty and start leaking.

Your consultant is correct; stainless steel units will repay their extra cost in savings because you will not need regular inspections and maintenance of the isolators on the roof.

“I require a few rotary switches but I can’t find the contact arrangement I need.”

We can supply a specially configured switch to suit your specific requirements. Just send us the sequence you need with details of the current rating and any special requirements such as a locking handle.

We will get back to you with a unique part number that you can order to get the contact arrangement that you require.

“I am tendering for an installation that needs a unit to automatically switch to a second supply should the normal supply fail. I am looking for something rated at 125 A for an essential pump supply.”

You need an Automatic Transfer Switch.

This detects that the normal supply has failed and will then automatically switch to the second supply

You can also have indicators to show which supply is being used and auxiliary contacts can be supplied to interface with a building management system.

The Automatic Transfer Switch will switch back to the normal supply when this is restored.

“I need an isolator with a current rating of 40 A but I am using 95 mm2 cable because this unit is at the end of a long cable run. Can you make a special unit with the rating I need that will accommodate the cable being used?”

Just use an isolator with a current rating related to the cable size which in your case would be 125 A

Using a higher rated device means that the isolator terminals will easily accept the cable you are using and there will be enough room in the enclosure to route the cable from the gland plate to the terminals.