Down Time

Imho this is a overestimated item. The major reason of this is, one: Several definitions are used and often it is not clear which one. second: Reliable figures depend on accurate administration. This accuracy is not always warranted

Definition:
In general there are two definitions for down time. From the patients point of view and for machine performance. For the patient it is important to be treated every day without long delays. Lets say one linac breaks down but the patient can be treated on an other linac (witch for example was just be serviced). In this case production is not inhibited and from this point of view there is now down time. On the other hand it is useful to have an impression of the performance of the linac itself. How many times does it break down. How long does it take to repair. Does one particular part of the linac give trouble (for example MLC). etc.
A point of discussion is if maintenance must been seen as down time or not. From the patients point of view the linac is not available so it would be down time. On the other hand when a linac is purchased it is known that maintenance must be carried out at regular intervals to ensure safe and trouble free operation. Seeing this maintenance as down time appears to me as bad management.

Administration:
We make extensive use of the debug terminal. All errors are registered automatically and at the end of the month my colleague edits the debug files and counts the number of interrupts. This is multiplied by a standard time factor of 5 min. When we replace lets say a light bulb we use a text editor available in the debug program to record the time we used. You can imagine that these manual entrees are sometimes not very accurate or even forgotten.

The definition we use is:
Down time is the time when a linac can not be used for treatment due to electrical, mechanical or physical faults during normal working hours. (This implies that maintenance during working hours and repairs in the evening or at weekends are not registered as down time).

Table 1: Down Time in percent. The average over 12 years is 2.4 percent, standard deviation is 1.1 percent. U1: mc5037 1987 SL25-15. U2: mc5047 1987 SL25-15. U3 until 1996 SL75-20 from 1999 mc5504 SL25-22 + MLC and Iview. U4: mc5087 1991 SL25-20 + MLC and Iview. U5: mc5074 1990 SL25-15. U6: mc5289 1995 SL25-15 + Iview

	U1	U2	U3	U4	U5	U6	Aver.
1989	1.7	5.7			7.8		5.1
1990	3.6	2.8		4.8	2.3		3.4
1991	0.6	2.3	1.0	1.5	3.2		1.7
1992	1.1	1.6	1.4	3.8	1.5		1.9
1993	0.8	4.3	1.4	0.7	6.5		2.7
1994	1.6	0.6	0.7	2.1	0.6		1.1
1995	1.6	3.1	0.4	0.6	2.1	0.6	1.4
1996	1.2	1.9	0.8	2.9	0.7	0.5	1.3
1997	2.0	5.4		1.7	1.6	4.6	3.1
1998	1.4	1.7		1.8	1.4	1.0	1.5
1999	1.4	0.9	5.7	6.5	0.6	2.1	2.7
2000	3.1	1.1	1.4	3.4	2.0	1.2	3.0

Some remarks.

• U1 1989 no data, 2000 vacuum problems and short circuit in bending magnet.
• U2 1989 no data, 1993 vacuum problems, 1995 hard-disk problems, 1997 SF6 leak in RF window.
• U3 1999 Prototype table.
• U4 1990 faulty charging transformer, 1992 replacement of diaphragm bearing, 1999 MLC camera trouble and replacement of reflectors, 2000 cleaning and calibrating MLC, replacing thyratron magnetron and ionisation chamber.
• U5 1989 no data, 1993 replacement flight tube and bending mag. problems, 2000 replacement of diaphragm bearing.
• U6 1997 RAM spindle nut replacement.

Courtesy of Cees Imhof