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Do I need a DOL for compressor

Alan B

Looking at installing this compressor, wonder how to interpret the instructions, do I need a DOL.

https://dccf75d8gej24.cloudfront.net/documents/(000112350)_XE29-270%20Air%20Compressor_ISS%205.pdf

specifications etc

https://www.machinemart.co.uk/p/clarke-xe29270---industrial-air-compressor-230/ 

All the instructions say about electrical installation is 

The power cable from the main supply must be large enough to carry the starting and running load of the electric motor. This is particularly relevant if the compressor is some distance from the source of supply. 6. Electrical installations should be completed by a qualified electrician. 7. Electrical connection to the mains supply must be via a suitably fused (see table on page 6) approved plug or isolator (allowing sufficient capacity for motor starting). If using a circuit breaker, ensure it is motor rated and of sufficient size to allow for motor starting.

Then later it has the table below for DOL ratings. 

But the instructions don't say there needs to be a DOL starter, so can I take that it's not required but if you want one these are the specifications.

The compressor is running a sand blasting cabinet in a domestic garage, the only moving parts are really the motors on the compressor that have good guards on.

The only real risk I can see is if someone decided to work on the compressor with the guards off. if someone was caught in the motor the only places reachable would be on the floor or mounted on the compressor, i don't fancy doing either.

The current plan is to have a 32a socket with built rotary isolator but no DOL. The compressor has two 3hp motors and sequential starting so that they can run on a 30A supply, as per other post I am using a 32A type C RCBO having eventually found one.

  • 0

    A DOL starter would be my preferred method. I would recommend that any item of plant with a motor rated higher than 0.75KW should be controlled via a DOL starter with a dedicated supply.

  • 0

    552.1.1 states that overload protection should be provided for a motor exceeding 0.37kW. The best (and most practical) means of achieving that for a relatively small motor is indeed a DOL starter with an overload. So just replace the rotary isolator with a small DOL starter and you satisfy local isolation as well as overload protection. It certainly isn't large enough to start considering star/delta, VSD or soft start. 

  • 0

    Oddly we have one like this - there  is an overload trip built into the lump on the motor, connecting via  with the pressure unloading valve, so that it does not try to start with  back pressure - a simple plug and socket would be OK, but be aware that if power is cut while it still pumping and then comes back on without being reset on the red button, it will stall, and the overheat trip operates.

  • 0

    A type "C" MCB/RCBO will probably trip when the motor starts. You should use type "D" (motor rated) MCBs for motors of this size.

    regards, burn

  • 0 in reply to burn

    Thanks for the heads up. I will try the type C and see how it works. Moving to a type D will mean changing the consumer unit or mixing manufacturers.

  • 0

    Your DOL isn't going to be able to be small enough to protect the individual motors. That's why they have the thermal overload built into the pressure switches. I think you are overthinking it a bit. I would hard wire it into an isolator if there's not much chance of it being moved. Then if you need a D MCB (very possible) you'll find one of those easier than a RCBO  and won't have to change the board

  • 0

    type "D" (motor rated)

    I was under the impression that C-types were quite usual for motors - the rarer D types bing used for things with even higher inrush currents such as transformers and X-ray machines. In any event the tripping level is a feature of the rating as well as the type - a C16 would be less likely to trip than a D6 for instance. If you have an overload at the motor then the MCB at the DB need only be rated for fault protection - so can have a significantly higher rating than either the motors steady state demand or the wiring rating would normally suggest.

       - Andy.

  • 0 in reply to AJJewsbury

    Agree, our 3hp Clarke one is on a C20 on a 4mm radial (length reasons). The overload trip on the motor does the close protection. It did have a 13A fused spur when first installed, but this ate fuses in cold weather.
    Mike

  • 0 in reply to mapj1

    Please don't tell anybody, but ...

    My 3 hp compressor chews up and spits out 13 A fuses too. However, a short length of 15 A fuse wire wrapped around a dead BS 1362 fuse has lasted for many years. Problem is that I am unable to use an extension lead.

    Now then boys and girls, please don't do what my chum did, which was to turn up a nice little brass rod exactly the same size as a fuse.

  • 0 in reply to Chris Pearson

    The Chinese will quite happily sell you fuses marked 15A and BS1362.  It's anyone's guess what will be inside them, though.

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